Revised5 Report on the Algorithmic Language Scheme
RICHARD KELSEY, WILLIAM CLINGER, AND JONATHAN REES (Editors)
H. ABELSON
N. I. ADAMS IV D. H. BARTLEY G. BROOKS
R. K. DYBVIG D. P. FRIEDMAN R. HALSTEAD
C. HANSON
C. T. HAYNES E. KOHLBECKER D. OXLEY
K. M. PITMAN
20 February 1998
SUMMARY
The report gives a defining description of the program- ming language Scheme. Scheme is a statically scoped and properlytail-recursivedialectoftheLispprogramming language invented by Guy Lewis Steele Jr. and Gerald Jay Sussman. It was designed to have an exceptionally clear and simple semantics and few different ways to form expressions. A wide variety of programming paradigms, in- cluding imperative, functional, and message passing styles, find convenient expression in Scheme.
The introduction offers a brief history of the language and of the report.
The first three chapters present the fundamental ideas of the language and describe the notational conventions used for describing the language and for writing programs in the language.
Chapters 4 and 5 describe the syntax and semantics of expressions, programs, and definitions.
Chapter 6 describes Scheme’s built-in procedures, which include all of the language’s data manipulation and in- put/output primitives.
Chapter 7 provides a formal syntax for Scheme written in extended BNF, along with a formal denotational semantics. An example of the use of the language follows the formal syntax and semantics.
The report concludes with a list of references and an al- phabetic index.
Dedicated to the Memory of Robert Hieb
G. J. ROZAS
G. L. STEELE JR. G. J. SUSSMAN M. WAND
CONTENTS
Introduction ………………………. 2 1 OverviewofScheme………………….. 3 1.1 Semantics ……………………. 3 1.2 Syntax……………………… 3 1.3 Notationandterminology ……………. 3 2 Lexicalconventions ………………….. 5 2.1 Identifiers ……………………. 5 2.2 Whitespaceandcomments……………. 5 2.3 Othernotations…………………. 5 3 Basicconcepts…………………….. 6
3.1 Variables, syntactic keywords, and regions . . . . . . . 6
3.2 Disjointnessoftypes ………………. 6
3.3 Externalrepresentations…………….. 6
3.4 Storagemodel………………….. 7
3.5 Propertailrecursion ………………. 7
4 Expressions ……………………… 8 4.1 Primitiveexpressiontypes……………. 8 4.2 Derivedexpressiontypes…………….. 10 4.3 Macros……………………… 13 5 Programstructure…………………… 16 5.1 Programs ……………………. 16 5.2 Definitions……………………. 16 5.3 Syntaxdefinitions ……………….. 17 6 Standardprocedures …………………. 17 6.1 Equivalencepredicates……………… 17 6.2 Numbers…………………….. 19 6.3 Otherdatatypes ………………… 25 6.4 Controlfeatures…………………. 31 6.5 Eval………………………. 35 6.6 Inputandoutput………………… 35 7 Formalsyntaxandsemantics……………… 38 7.1 Formalsyntax………………….. 38 7.2 Formalsemantics………………… 40 7.3 Derivedexpressiontypes…………….. 43 Notes ………………………….. 45 Additionalmaterial …………………… 45 Example…………………………. 45 References………………………… 46
Alphabetic index of definitions of concepts, keywords,andprocedures ……………. 48
2 Revised5 Scheme
Programming languages should be designed not by piling feature on top of feature, but by removing the weaknesses and restrictions that make additional features appear nec- essary. Scheme demonstrates that a very small number of rules for forming expressions, with no restrictions on how they are composed, suffice to form a practical and efficient programming language that is flexible enough to support most of the major programming paradigms in use today.
Scheme was one of the first programming languages to in- corporate first class procedures as in the lambda calculus, thereby proving the usefulness of static scope rules and block structure in a dynamically typed language. Scheme was the first major dialect of Lisp to distinguish procedures from lambda expressions and symbols, to use a single lex- ical environment for all variables, and to evaluate the op- erator position of a procedure call in the same way as an operand position. By relying entirely on procedure calls to express iteration, Scheme emphasized the fact that tail- recursive procedure calls are essentially goto’s that pass arguments. Scheme was the first widely used program- ming language to embrace first class escape procedures, from which all previously known sequential control struc- tures can be synthesized. A subsequent version of Scheme introduced the concept of exact and inexact numbers, an extension of Common Lisp’s generic arithmetic. More re- cently, Scheme became the first programming language to support hygienic macros, which permit the syntax of a block-structured language to be extended in a consistent and reliable manner.
Background
The first description of Scheme was written in 1975 [28]. A revised report [25] appeared in 1978, which described the evolution of the language as its MIT implementation was upgraded to support an innovative compiler [26]. Three distinct projects began in 1981 and 1982 to use variants of Scheme for courses at MIT, Yale, and Indiana Univer- sity [21, 17, 10]. An introductory computer science text- book using Scheme was published in 1984 [1].
As Scheme became more widespread, local dialects be- gan to diverge until students and researchers occasion- ally found it difficult to understand code written at other sites. Fifteen representatives of the major implementations of Scheme therefore met in October 1984 to work toward a better and more widely accepted standard for Scheme. Their report [4] was published at MIT and Indiana Uni- versity in the summer of 1985. Further revision took place in the spring of 1986 [23], and in the spring of 1988 [6]. The present report reflects further revisions agreed upon in a meeting at Xerox PARC in June 1992.
We intend this report to belong to the entire Scheme com-
munity, and so we grant permission to copy it in whole or in part without fee. In particular, we encourage implementors of Scheme to use this report as a starting point for manuals and other documentation, modifying it as necessary.
Acknowledgements
We would like to thank the following people for their help: Alan Bawden, Michael Blair, George Carrette, Andy Cromarty, Pavel Curtis, Jeff Dalton, Olivier Danvy, Ken Dickey, Bruce Duba, Marc Feeley, Andy Freeman, Richard Gabriel, Yekta Gu ̈rsel, Ken Haase, Robert Hieb, Paul Hudak, Morry Katz, Chris Lindblad, Mark Meyer, Jim Miller, Jim Philbin, John Ramsdell, Mike Shaff, Jonathan Shapiro, Julie Sussman, Perry Wagle, Daniel Weise, Henry Wu, and Ozan Yigit. We thank Carol Fessenden, Daniel Friedman, and Christopher Haynes for permission to use text from the Scheme 311 version 4 reference manual. We thank Texas Instruments, Inc. for permission to use text from the TI Scheme Language Reference Manual [30]. We gladly acknowledge the influence of manuals for MIT Scheme [17], T [22], Scheme 84 [11], Common Lisp [27], and Algol 60 [18].
We also thank Betty Dexter for the extreme effort she put into setting this report in TEX, and Donald Knuth for de- signing the program that caused her troubles.
The Artificial Intelligence Laboratory of the Massachusetts Institute of Technology, the Computer Science Department of Indiana University, the Computer and Information Sci- ences Department of the University of Oregon, and the NEC Research Institute supported the preparation of this report. Support for the MIT work was provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014- 80-C-0505. Support for the Indiana University work was provided by NSF grants NCS 83-04567 and NCS 83-03325.
INTRODUCTION
1. Overview of Scheme 1.1. Semantics
This section gives an overview of Scheme’s semantics. A detailed informal semantics is the subject of chapters 3 through 6. For reference purposes, section 7.2 provides a formal semantics of Scheme.
Following Algol, Scheme is a statically scoped program- ming language. Each use of a variable is associated with a lexically apparent binding of that variable.
Scheme has latent as opposed to manifest types. Types are associated with values (also called objects) rather than with variables. (Some authors refer to languages with latent types as weakly typed or dynamically typed lan- guages.) Other languages with latent types are APL, Snobol, and other dialects of Lisp. Languages with mani- fest types (sometimes referred to as strongly typed or stat- ically typed languages) include Algol 60, Pascal, and C.
All objects created in the course of a Scheme computation, including procedures and continuations, have unlimited ex- tent. No Scheme object is ever destroyed. The reason that implementations of Scheme do not (usually!) run out of storage is that they are permitted to reclaim the storage occupied by an object if they can prove that the object cannot possibly matter to any future computation. Other languages in which most objects have unlimited extent in- clude APL and other Lisp dialects.
Implementations of Scheme are required to be properly tail-recursive. This allows the execution of an iterative computation in constant space, even if the iterative compu- tation is described by a syntactically recursive procedure. Thus with a properly tail-recursive implementation, iter- ation can be expressed using the ordinary procedure-call mechanics, so that special iteration constructs are useful only as syntactic sugar. See section 3.5.
Scheme procedures are objects in their own right. Pro- cedures can be created dynamically, stored in data struc- tures, returned as results of procedures, and so on. Other languages with these properties include Common Lisp and ML.
One distinguishing feature of Scheme is that continuations, which in most other languages only operate behind the scenes, also have “first-class” status. Continuations are useful for implementing a wide variety of advanced control constructs, including non-local exits, backtracking, and coroutines. See section 6.4.
Arguments to Scheme procedures are always passed by value, which means that the actual argument expressions are evaluated before the procedure gains control, whether the procedure needs the result of the evaluation or not.
ML, C, and APL are three other languages that always pass arguments by value. This is distinct from the lazy- evaluation semantics of Haskell, or the call-by-name se- mantics of Algol 60, where an argument expression is not evaluated unless its value is needed by the procedure.
Scheme’s model of arithmetic is designed to remain as in- dependent as possible of the particular ways in which num- bers are represented within a computer. In Scheme, every integer is a rational number, every rational is a real, and every real is a complex number. Thus the distinction be- tween integer and real arithmetic, so important to many programming languages, does not appear in Scheme. In its place is a distinction between exact arithmetic, which cor- responds to the mathematical ideal, and inexact arithmetic on approximations. As in Common Lisp, exact arithmetic is not limited to integers.
1.2. Syntax
Scheme, like most dialects of Lisp, employs a fully paren- thesized prefix notation for programs and (other) data; the grammar of Scheme generates a sublanguage of the lan- guage used for data. An important consequence of this sim- ple, uniform representation is the susceptibility of Scheme programs and data to uniform treatment by other Scheme programs. For example, the eval procedure evaluates a Scheme program expressed as data.
The read procedure performs syntactic as well as lexical decomposition of the data it reads. The read procedure parses its input as data (section 7.1.2), not as program.
The formal syntax of Scheme is described in section 7.1. 1.3. Notation and terminology
1.3.1. Primitive, library, and optional features
It is required that every implementation of Scheme support all features that are not marked as being optional. Imple- mentations are free to omit optional features of Scheme or to add extensions, provided the extensions are not in conflict with the language reported here. In particular, implementations must support portable code by providing a syntactic mode that preempts no lexical conventions of this report.
To aid in understanding and implementing Scheme, some features are marked as library. These can be easily imple- mented in terms of the other, primitive, features. They are redundant in the strict sense of the word, but they capture common patterns of usage, and are therefore provided as convenient abbreviations.
DESCRIPTION OF THE LANGUAGE
1. Overview of Scheme 3
4 Revised5 Scheme
1.3.2. Error situations and unspecified behavior
When speaking of an error situation, this report uses the phrase “an error is signalled” to indicate that implemen- tations must detect and report the error. If such wording does not appear in the discussion of an error, then imple- mentations are not required to detect or report the error, though they are encouraged to do so. An error situation that implementations are not required to detect is usually referred to simply as “an error.”
For example, it is an error for a procedure to be passed an argument that the procedure is not explicitly specified to handle, even though such domain errors are seldom men- tioned in this report. Implementations may extend a pro- cedure’s domain of definition to include such arguments.
This report uses the phrase “may report a violation of an implementation restriction” to indicate circumstances un- der which an implementation is permitted to report that it is unable to continue execution of a correct program be- cause of some restriction imposed by the implementation. Implementation restrictions are of course discouraged, but implementations are encouraged to report violations of im- plementation restrictions.
For example, an implementation may report a violation of an implementation restriction if it does not have enough storage to run a program.
If the value of an expression is said to be “unspecified,” then the expression must evaluate to some object without signalling an error, but the value depends on the imple- mentation; this report explicitly does not say what value should be returned.
1.3.3. Entry format
Chapters 4 and 6 are organized into entries. Each entry de- scribes one language feature or a group of related features, where a feature is either a syntactic construct or a built-in procedure. An entry begins with one or more header lines of the form
⟨thing1⟩ . . .
indicates zero or more occurrences of a ⟨thing⟩, and
⟨thing1⟩ ⟨thing2⟩ . . .
indicates one or more occurrences of a ⟨thing⟩.
If category is “procedure”, then the entry describes a pro- cedure, and the header line gives a template for a call to the procedure. Argument names in the template are italicized. Thus the header line
template
for required, primitive features, or
1.3.4. Evaluation examples
The symbol “=⇒” used in program examples should be read “evaluates to.” For example,
(*58) =⇒ 40
means that the expression (* 5 8) evaluates to the ob- ject 40. Or, more precisely: the expression given by the sequence of characters “(* 5 8)” evaluates, in the initial environment, to an object that may be represented exter- nally by the sequence of characters “40”. See section 3.3 for a discussion of external representations of objects.
1.3.5. Naming conventions
By convention, the names of procedures that always return a boolean value usually end in “?”. Such procedures are called predicates.
category qualifier category
If category is “syntax”, the entry describes an expression type, and the template gives the syntax of the expression type. Components of expressions are designated by syn- tactic variables, which are written using angle brackets, for example, ⟨expression⟩, ⟨variable⟩. Syntactic variables should be understood to denote segments of program text; for example, ⟨expression⟩ stands for any string of charac- ters which is a syntactically valid expression. The notation
template
where qualifier is either “library” or “optional” as defined
in section 1.3.1.
(vector-ref vector k)
procedure
indicates that the built-in procedure vector-ref takes two arguments, a vector vector and an exact non-negative in- teger k (see below). The header lines
(make-vector k) (make-vector k fill)
procedure procedure
indicate that the make-vector procedure must be defined to take either one or two arguments.
It is an error for an operation to be presented with an ar- gument that it is not specified to handle. For succinctness, we follow the convention that if an argument name is also the name of a type listed in section 3.2, then that argu- ment must be of the named type. For example, the header line for vector-ref given above dictates that the first ar- gument to vector-ref must be a vector. The following naming conventions also imply type restrictions:
obj
list, list1, … listj, … z, z1, … zj, …
x, x1, … xj, …
y, y1, … yj, …
q, q1, … qj, … n, n1, … nj, … k, k1, … kj, …
any object
list (see section 6.3.2) complex number
real number
real number
rational number
integer
exact non-negative integer
By convention, the names of procedures that store values into previously allocated locations (see section 3.4) usually end in “!”. Such procedures are called mutation proce- dures. By convention, the value returned by a mutation procedure is unspecified.
By convention, “->” appears within the names of proce- dures that take an object of one type and return an anal- ogous object of another type. For example, list->vector takes a list and returns a vector whose elements are the same as those of the list.
2. Lexical conventions
This section gives an informal account of some of the lexical conventions used in writing Scheme programs. For a formal syntax of Scheme, see section 7.1.
Upper and lower case forms of a letter are never distin- guished except within character and string constants. For example, Foo is the same identifier as FOO, and #x1AB is the same number as #X1ab.
2.1. Identifiers
Most identifiers allowed by other programming languages are also acceptable to Scheme. The precise rules for form- ing identifiers vary among implementations of Scheme, but in all implementations a sequence of letters, digits, and “ex- tended alphabetic characters” that begins with a character that cannot begin a number is an identifier. In addition, +, -, and … are identifiers. Here are some examples of identifiers:
lambda q
list->vector soup
+ V17a
<=? a34kTMNs
the-word-recursion-has-many-meanings
Extended alphabetic characters may be used within iden- tifiers as if they were letters. The following are extended alphabetic characters:
!$%&*+-./:<=>?@^_~
See section 7.1.1 for a formal syntax of identifiers. Identifiers have two uses within Scheme programs:
• Any identifier may be used as a variable or as a syn- tactic keyword (see sections 3.1 and 4.3).
• When an identifier appears as a literal or within a literal (see section 4.1.2), it is being used to denote a symbol (see section 6.3.3).
2.2. Whitespace and comments
Whitespace characters are spaces and newlines. (Imple- mentations typically provide additional whitespace char- acters such as tab or page break.) Whitespace is used for improved readability and as necessary to separate tokens from each other, a token being an indivisible lexical unit such as an identifier or number, but is otherwise insignifi- cant. Whitespace may occur between any two tokens, but not within a token. Whitespace may also occur inside a string, where it is significant.
A semicolon (;) indicates the start of a comment. The comment continues to the end of the line on which the semicolon appears. Comments are invisible to Scheme, but the end of the line is visible as whitespace. This prevents a comment from appearing in the middle of an identifier or number.
;;; The FACT procedure computes the factorial
;;; of a non-negative integer.
(define fact
(lambda (n)
(if (= n 0)
1 ;Base case: return 1
(* n (fact (- n 1))))))
2.3. Other notations
For a description of the notations used for numbers, see section 6.2.
. + – These are used in numbers, and may also occur anywhere in an identifier except as the first charac- ter. A delimited plus or minus sign by itself is also an identifier. A delimited period (not occurring within a number or identifier) is used in the notation for pairs (section 6.3.2), and to indicate a rest-parameter in a formal parameter list (section 4.1.4). A delimited se- quence of three successive periods is also an identifier.
( ) Parentheses are used for grouping and to notate lists (section 6.3.2).
’ The single quote character is used to indicate literal data (section 4.1.2).
` The backquote character is used to indicate almost- constant data (section 4.2.6).
, ,@ The character comma and the sequence comma at- sign are used in conjunction with backquote (sec- tion 4.2.6).
” The double quote character is used to delimit strings (section 6.3.5).
2. Lexical conventions 5
6 Revised5 Scheme
\ Backslash is used in the syntax for character constants (section 6.3.4) and as an escape character within string constants (section 6.3.5).
[ ] { } | Left and right square brackets and curly braces and vertical bar are reserved for possible future exten- sions to the language.
# Sharp sign is used for a variety of purposes depending on the character that immediately follows it:
#t #f These are the boolean constants (section 6.3.1).
#\ This introduces a character constant (section 6.3.4).
#( This introduces a vector constant (section 6.3.6). Vec- tor constants are terminated by ) .
#e #i #b #o #d #x These are used in the notation for numbers (section 6.2.4).
The region is determined by the particular binding con- struct that establishes the binding; if the binding is estab- lished by a lambda expression, for example, then its region is the entire lambda expression. Every mention of an iden- tifier refers to the binding of the identifier that established the innermost of the regions containing the use. If there is no binding of the identifier whose region contains the use, then the use refers to the binding for the variable in the top level environment, if any (chapters 4 and 6); if there is no binding for the identifier, it is said to be unbound.
3.2. Disjointness of types
No object satisfies more than one of the following predi- cates:
3.
Basic concepts
3.1. Variables, syntactic keywords, and re- gions
An identifier may name a type of syntax, or it may name a location where a value can be stored. An identifier that names a type of syntax is called a syntactic keyword and is said to be bound to that syntax. An identifier that names a location is called a variable and is said to be bound to that location. The set of all visible bindings in effect at some point in a program is known as the environment in effect at that point. The value stored in the location to which a variable is bound is called the variable’s value. By abuse of terminology, the variable is sometimes said to name the value or to be bound to the value. This is not quite accurate, but confusion rarely results from this practice.
Certain expression types are used to create new kinds of syntax and bind syntactic keywords to those new syntaxes, while other expression types create new locations and bind variables to those locations. These expression types are called binding constructs. Those that bind syntactic key- words are listed in section 4.3. The most fundamental of the variable binding constructs is the lambda expression, because all other variable binding constructs can be ex- plained in terms of lambda expressions. The other variable binding constructs are let, let*, letrec, and do expres- sions (see sections 4.1.4, 4.2.2, and 4.2.4).
Like Algol and Pascal, and unlike most other dialects of Lisp except for Common Lisp, Scheme is a statically scoped language with block structure. To each place where an identifier is bound in a program there corresponds a region of the program text within which the binding is visible.
These predicates define the types boolean, pair, symbol, number, char (or character), string, vector, port, and pro- cedure. The empty list is a special object of its own type; it satisfies none of the above predicates.
Although there is a separate boolean type, any Scheme value can be used as a boolean value for the purpose of a conditional test. As explained in section 6.3.1, all values count as true in such a test except for #f. This report uses the word “true” to refer to any Scheme value except #f, and the word “false” to refer to #f.
3.3. External representations
An important concept in Scheme (and Lisp) is that of the external representation of an object as a sequence of char- acters. For example, an external representation of the inte- ger 28 is the sequence of characters “28”, and an external representation of a list consisting of the integers 8 and 13 is the sequence of characters “(8 13)”.
The external representation of an object is not neces- sarily unique. The integer 28 also has representations “#e28.000” and “#x1c”, and the list in the previous para- graph also has the representations “( 08 13 )” and “(8 . (13 . ()))” (see section 6.3.2).
Many objects have standard external representations, but some, such as procedures, do not have standard represen- tations (although particular implementations may define representations for them).
An external representation may be written in a program to obtain the corresponding object (see quote, section 4.1.2).
boolean?
symbol?
char?
vector?
procedure?
pair?
number?
string?
port?
External representations can also be used for input and output. The procedure read (section 6.6.2) parses ex- ternal representations, and the procedure write (sec- tion 6.6.3) generates them. Together, they provide an elegant and powerful input/output facility.
Note that the sequence of characters “(+ 2 6)” is not an external representation of the integer 8, even though it is an expression evaluating to the integer 8; rather, it is an exter- nal representation of a three-element list, the elements of which are the symbol + and the integers 2 and 6. Scheme’s syntax has the property that any sequence of characters that is an expression is also the external representation of some object. This can lead to confusion, since it may not be obvious out of context whether a given sequence of char- acters is intended to denote data or program, but it is also a source of power, since it facilitates writing programs such as interpreters and compilers that treat programs as data (or vice versa).
The syntax of external representations of various kinds of objects accompanies the description of the primitives for manipulating the objects in the appropriate sections of chapter 6.
3.4. Storage model
Variables and objects such as pairs, vectors, and strings implicitly denote locations or sequences of locations. A string, for example, denotes as many locations as there are characters in the string. (These locations need not correspond to a full machine word.) A new value may be stored into one of these locations using the string-set! procedure, but the string continues to denote the same locations as before.
An object fetched from a location, by a variable reference or by a procedure such as car, vector-ref, or string-ref, is equivalent in the sense of eqv? (section 6.1) to the object last stored in the location before the fetch.
Every location is marked to show whether it is in use. No variable or object ever refers to a location that is not in use. Whenever this report speaks of storage being allocated for a variable or object, what is meant is that an appropriate number of locations are chosen from the set of locations that are not in use, and the chosen locations are marked to indicate that they are now in use before the variable or object is made to denote them.
In many systems it is desirable for constants (i.e. the val- ues of literal expressions) to reside in read-only-memory. To express this, it is convenient to imagine that every object that denotes locations is associated with a flag telling whether that object is mutable or immutable. In such systems literal constants and the strings returned by symbol->string are immutable objects, while all objects
created by the other procedures listed in this report are mutable. It is an error to attempt to store a new value into a location that is denoted by an immutable object.
3.5. Proper tail recursion
Implementations of Scheme are required to be properly tail- recursive. Procedure calls that occur in certain syntactic contexts defined below are ‘tail calls’. A Scheme imple- mentation is properly tail-recursive if it supports an un- bounded number of active tail calls. A call is active if the called procedure may still return. Note that this in- cludes calls that may be returned from either by the cur- rent continuation or by continuations captured earlier by call-with-current-continuation that are later invoked. In the absence of captured continuations, calls could return at most once and the active calls would be those that had not yet returned. A formal definition of proper tail recur- sion can be found in [8].
Rationale:
Intuitively, no space is needed for an active tail call because the continuation that is used in the tail call has the same semantics as the continuation passed to the procedure containing the call. Although an improper implementation might use a new con- tinuation in the call, a return to this new continuation would be followed immediately by a return to the continuation passed to the procedure. A properly tail-recursive implementation re- turns to that continuation directly.
Proper tail recursion was one of the central ideas in Steele and Sussman’s original version of Scheme. Their first Scheme in- terpreter implemented both functions and actors. Control flow was expressed using actors, which differed from functions in that they passed their results on to another actor instead of returning to a caller. In the terminology of this section, each actor finished with a tail call to another actor.
Steele and Sussman later observed that in their interpreter the code for dealing with actors was identical to that for functions and thus there was no need to include both in the language.
A tail call is a procedure call that occurs in a tail con- text. Tail contexts are defined inductively. Note that a tail context is always determined with respect to a particular lambda expression.
• The last expression within the body of a lambda ex- pression, shown as ⟨tail expression⟩ below, occurs in a tail context.
(lambda ⟨formals⟩
⟨definition⟩* ⟨expression⟩* ⟨tail expression⟩)
• If one of the following expressions is in a tail context, then the subexpressions shown as ⟨tail expression⟩ are in a tail context. These were derived from rules in
3. Basic concepts 7
8 Revised5 Scheme
the grammar given in chapter 7 by replacing some oc-
currences of ⟨expression⟩ with ⟨tail expression⟩. Only those rules that contain tail contexts are shown here.
(if ⟨expression⟩ ⟨tail expression⟩ ⟨tail expression⟩) (if ⟨expression⟩ ⟨tail expression⟩)
(cond ⟨cond clause⟩+)
(cond ⟨cond clause⟩* (else ⟨tail sequence⟩))
(case ⟨expression⟩ ⟨case clause⟩+) (case ⟨expression⟩
⟨case clause⟩*
(else ⟨tail sequence⟩))
(and ⟨expression⟩* ⟨tail expression⟩) (or ⟨expression⟩* ⟨tail expression⟩)
(let (⟨binding spec⟩*) ⟨tail body⟩)
(let ⟨variable⟩ (⟨binding spec⟩*) ⟨tail body⟩) (let* (⟨binding spec⟩*) ⟨tail body⟩)
(letrec (⟨binding spec⟩*) ⟨tail body⟩)
(let-syntax (⟨syntax spec⟩*) ⟨tail body⟩) (letrec-syntax (⟨syntax spec⟩*) ⟨tail body⟩)
(begin ⟨tail sequence⟩)
(do (⟨iteration spec⟩*) (⟨test⟩ ⟨tail sequence⟩)
⟨expression⟩*) where
⟨cond clause⟩ −→ (⟨test⟩ ⟨tail sequence⟩)
⟨case clause⟩ −→ ((⟨datum⟩*) ⟨tail sequence⟩)
⟨tail body⟩ −→ ⟨definition⟩* ⟨tail sequence⟩
⟨tail sequence⟩ −→ ⟨expression⟩* ⟨tail expression⟩
• If a cond expression is in a tail context, and has a clause of the form (⟨expression1⟩ => ⟨expression2⟩) then the (implied) call to the procedure that results from the evaluation of ⟨expression2⟩ is in a tail context. ⟨expression2⟩ itself is not in a tail context.
Certain built-in procedures are also required to perform tail calls. The first argument passed to apply and to call-with-current-continuation, and the second argu- ment passed to call-with-values, must be called via a tail call. Similarly, eval must evaluate its argument as if it were in tail position within the eval procedure.
In the following example the only tail call is the call to f. None of the calls to g or h are tail calls. The reference to x is in a tail context, but it is not a call and thus is not a tail call.
(lambda ()
(if (g)
(let ((x (h)))
x)
(and (g) (f))))
Note:
4. Expressions
Expression types are categorized as primitive or derived. Primitive expression types include variables and procedure calls. Derived expression types are not semantically prim- itive, but can instead be defined as macros. With the ex- ception of quasiquote, whose macro definition is complex, the derived expressions are classified as library features. Suitable definitions are given in section 7.3.
Implementations are allowed, but not required, to recog- nize that some non-tail calls, such as the call to h above, can be evaluated as though they were tail calls. In the example above, the let expression could be compiled as a tail call to h. (The possibility of h returning an unexpected number of values can be ignored, because in that case the effect of the let is explicitly unspecified and implementation-dependent.)
4.1. Primitive expression types
4.1.1. Variable references
⟨variable⟩
syntax
An expression consisting of a variable (section 3.1) is a variable reference. The value of the variable reference is the value stored in the location to which the variable is bound. It is an error to reference an unbound variable.
(define x 28)
x
4.1.2. Literal expressions
(quote ⟨datum⟩) ’⟨datum⟩ ⟨constant⟩
=⇒ 28
(quote ⟨datum⟩) evaluates to ⟨datum⟩. ⟨Datum⟩ may be any external representation of a Scheme object (see sec- tion 3.3). This notation is used to include literal constants in Scheme code.
(quotea)
(quote #(a b c)) (quote (+ 1 2))
=⇒ a
=⇒ #(a b c) =⇒ (+ 1 2)
syntax syntax syntax
(quote ⟨datum⟩) may be abbreviated as ’⟨datum⟩. The two notations are equivalent in all respects.
Note: In many dialects of Lisp, the empty combination, (), is a legitimate expression. In Scheme, combinations must have at least one subexpression, so () is not a syntactically valid expression.
’a
’#(a b c)
’()
’(+ 1 2)
’(quote a)
’’a
=⇒a =⇒#(abc) =⇒()
=⇒ (+12) =⇒ (quote a) =⇒ (quote a)
4.1.4. Procedures
(lambda ⟨formals⟩ ⟨body⟩)
syntax
Numerical constants, string constants, character constants, and boolean constants evaluate “to themselves”; they need not be quoted.
Syntax: ⟨Formals⟩ should be a formal arguments list as described below, and ⟨body⟩ should be a sequence of one or more expressions.
Semantics: A lambda expression evaluates to a procedure. The environment in effect when the lambda expression was evaluated is remembered as part of the procedure. When the procedure is later called with some actual arguments, the environment in which the lambda expression was evalu- ated will be extended by binding the variables in the formal argument list to fresh locations, the corresponding actual argument values will be stored in those locations, and the expressions in the body of the lambda expression will be evaluated sequentially in the extended environment. The result(s) of the last expression in the body will be returned as the result(s) of the procedure call.
’”abc” =⇒ “abc” =⇒ ’145932 =⇒ 145932 =⇒ ’#t =⇒ #t =⇒
“abc”
“abc”
145932
145932
#t
#t
As noted in section 3.4, it is an error to alter a constant (i.e. the value of a literal expression) using a mutation pro- cedure like set-car! or string-set!.
4.1.3. Procedure calls
(⟨operator⟩ ⟨operand1⟩ …)
syntax
(lambda (x) (+ x x))
((lambda (x) (+ x x)) 4)
(define reverse-subtract
(lambda (x y) (- y x)))
(reverse-subtract 7 10)
(define add4
(let ((x 4))
=⇒ a procedure =⇒ 8
=⇒ 3
A procedure call is written by simply enclosing in paren- theses expressions for the procedure to be called and the arguments to be passed to it. The operator and operand expressions are evaluated (in an unspecified order) and the resulting procedure is passed the resulting arguments.
(+34) =⇒ 7 ((if#f+*)34) =⇒ 12
A number of procedures are available as the values of vari- ables in the initial environment; for example, the addition and multiplication procedures in the above examples are the values of the variables + and *. New procedures are cre- ated by evaluating lambda expressions (see section 4.1.4).
Procedure calls may return any number of values (see values in section 6.4). With the exception of values the procedures available in the initial environment return one value or, for procedures such as apply, pass on the values returned by a call to one of their arguments.
Procedure calls are also called combinations.
Note: In contrast to other dialects of Lisp, the order of evaluation is unspecified, and the operator expression and the operand expressions are always evaluated with the same evalu- ation rules.
Note: Although the order of evaluation is otherwise unspeci- fied, the effect of any concurrent evaluation of the operator and operand expressions is constrained to be consistent with some sequential order of evaluation. The order of evaluation may be chosen differently for each procedure call.
(lambda (y) (+ x y)))) (add46)
=⇒ 10 ⟨Formals⟩ should have one of the following forms:
• (⟨variable1⟩ …): The procedure takes a fixed num- ber of arguments; when the procedure is called, the arguments will be stored in the bindings of the corre- sponding variables.
• ⟨variable⟩: The procedure takes any number of ar- guments; when the procedure is called, the sequence of actual arguments is converted into a newly allo- cated list, and the list is stored in the binding of the ⟨variable⟩.
• (⟨variable1 ⟩ . . . ⟨variablen ⟩ . ⟨variablen+1 ⟩): If a space-delimited period precedes the last variable, then the procedure takes n or more arguments, where n is the number of formal arguments before the period (there must be at least one). The value stored in the binding of the last variable will be a newly allocated list of the actual arguments left over after all the other actual arguments have been matched up against the other formal arguments.
4. Expressions 9
10 Revised5 Scheme
It is an error for a ⟨variable⟩ to appear more than once in
4.2.1. Conditionals
(cond ⟨clause1⟩ ⟨clause2⟩ . . . ) library syntax Syntax: Each ⟨clause⟩ should be of the form
(⟨test⟩ ⟨expression1⟩ …)
where ⟨test⟩ is any expression. Alternatively, a ⟨clause⟩
may be of the form
(⟨test⟩ => ⟨expression⟩)
The last ⟨clause⟩ may be an “else clause,” which has the
form
(else ⟨expression1⟩ ⟨expression2⟩ …).
Semantics: A cond expression is evaluated by evaluating
the ⟨test⟩ expressions of successive ⟨clause⟩s in order until one of them evaluates to a true value (see section 6.3.1). When a ⟨test⟩ evaluates to a true value, then the remain- ing ⟨expression⟩s in its ⟨clause⟩ are evaluated in order, and the result(s) of the last ⟨expression⟩ in the ⟨clause⟩ is(are) returned as the result(s) of the entire cond expres- sion. If the selected ⟨clause⟩ contains only the ⟨test⟩ and no ⟨expression⟩s, then the value of the ⟨test⟩ is returned as the result. If the selected ⟨clause⟩ uses the => alternate form, then the ⟨expression⟩ is evaluated. Its value must be a pro- cedure that accepts one argument; this procedure is then called on the value of the ⟨test⟩ and the value(s) returned by this procedure is(are) returned by the cond expression. If all ⟨test⟩s evaluate to false values, and there is no else clause, then the result of the conditional expression is un- specified; if there is an else clause, then its ⟨expression⟩s are evaluated, and the value(s) of the last one is(are) returned.
⟨formals⟩. ((lambdaxx)3456)
((lambda (x y . z) z) 3456)
=⇒ (3456) =⇒ (56)
Each procedure created as the result of evaluating a lambda expression is (conceptually) tagged with a storage location, in order to make eqv? and eq? work on procedures (see section 6.1).
4.1.5. Conditionals
(if ⟨test⟩ ⟨consequent⟩ ⟨alternate⟩) (if ⟨test⟩ ⟨consequent⟩)
syntax syntax
Syntax: ⟨Test⟩, ⟨consequent⟩, and ⟨alternate⟩ may be arbi- trary expressions.
Semantics: An if expression is evaluated as follows: first, ⟨test⟩ is evaluated. If it yields a true value (see sec- tion 6.3.1), then ⟨consequent⟩ is evaluated and its value(s) is(are) returned. Otherwise ⟨alternate⟩ is evaluated and its value(s) is(are) returned. If ⟨test⟩ yields a false value and no ⟨alternate⟩ is specified, then the result of the expression is unspecified.
(if (> 3 2) ’yes ’no)
(if (> 2 3) ’yes ’no)
(if (> 3 2)
(- 3 2) (+32))
4.1.6. Assignments
=⇒ yes =⇒ no
=⇒ 1
(set! ⟨variable⟩ ⟨expression⟩)
⟨Expression⟩ is evaluated, and the resulting value is stored
in the location to which ⟨variable⟩ is bound. ⟨Variable⟩ must be bound either in some region enclosing the set! expression or at top level. The result of the set! expression is unspecified.
(define x 2) (+x1) (set! x 4) (+x1)
=⇒ 3
=⇒ unspecified =⇒ 5
4.2. Derived expression types
The constructs in this section are hygienic, as discussed in section 4.3. For reference purposes, section 7.3 gives macro definitions that will convert most of the constructs described in this section into the primitive constructs de- scribed in the previous section.
syntax
=⇒ equal =⇒ 2
(case ⟨key⟩ ⟨clause1⟩
Syntax: ⟨Key⟩ may be any expression. Each ⟨clause⟩
should have the form
((⟨datum1⟩ …) ⟨expression1⟩ ⟨expression2⟩ …),
where each ⟨datum⟩ is an external representation of some object. All the ⟨datum⟩s must be distinct. The last ⟨clause⟩ may be an “else clause,” which has the form
(else ⟨expression1⟩ ⟨expression2⟩ …).
Semantics: A case expression is evaluated as follows. ⟨Key⟩ is evaluated and its result is compared against each ⟨datum⟩. If the result of evaluating ⟨key⟩ is equivalent (in the sense of eqv?; see section 6.1) to a ⟨datum⟩, then the expressions in the corresponding ⟨clause⟩ are evaluated from left to right and the result(s) of the last expression in
(cond ((> 3 2) ’greater)
((< 3 2) ’less))
(cond ((> 3 3) ’greater)
((< 3 3) ’less)
=⇒ greater
(else ’equal))
(cond ((assv ’b ’((a 1) (b 2))) => cadr)
(else #f))
⟨clause2⟩
. . . )
library syntax
the ⟨clause⟩ is(are) returned as the result(s) of the case ex- pression. If the result of evaluating ⟨key⟩ is different from every ⟨datum⟩, then if there is an else clause its expres- sions are evaluated and the result(s) of the last is(are) the result(s) of the case expression; otherwise the result of the case expression is unspecified.
initial values are being computed, thus allowing mutually recursive definitions.
(or
⟨test1⟩
. . . )
and ⟨body⟩ should be a sequence of one or more expres- sions.
Semantics: Let* is similar to let, but the bindings are performed sequentially from left to right, and the region of a binding indicated by (⟨variable⟩ ⟨init⟩) is that part of the let* expression to the right of the binding. Thus the second binding is done in an environment in which the first binding is visible, and so on.
(case (* 2 3)
((2 3 5 7) ’prime)
((1 4 6 8 9) ’composite)) =⇒
composite
unspecified
consonant
library syntax
((⟨variable1⟩ ⟨init1⟩) …),
where each ⟨init⟩ is an expression, and ⟨body⟩ should be a
sequence of one or more expressions. It is an error for a ⟨variable⟩ to appear more than once in the list of variables being bound.
Semantics: The ⟨init⟩s are evaluated in the current envi- ronment (in some unspecified order), the ⟨variable⟩s are bound to fresh locations holding the results, the ⟨body⟩ is evaluated in the extended environment, and the value(s) of the last expression of ⟨body⟩ is(are) returned. Each bind- ing of a ⟨variable⟩ has ⟨body⟩ as its region.
(case (car ’(c d))
((a) ’a)
((b) ’b))
(case (car ’(c d))
((a e i o u) ’vowel)
((w y) ’semivowel)
(else ’consonant))
(and ⟨test1⟩ . . . )
=⇒
=⇒
The ⟨test⟩ expressions are evaluated from left to right, and the value of the first expression that evaluates to a false value (see section 6.3.1) is returned. Any remaining ex- pressions are not evaluated. If all the expressions evaluate to true values, the value of the last expression is returned. If there are no expressions then #t is returned.
(let ((x 2) (y 3))
(*xy)) =⇒ 6
(let ((x 2) (y 3))
(let ((x 7)
(z (+ x y)))
(*zx))) =⇒ 35
See also named let, section 4.2.4. (let* ⟨bindings⟩ ⟨body⟩)
Syntax: ⟨Bindings⟩ should have the form ((⟨variable1⟩ ⟨init1⟩) …),
(and(=22)(>21)) (and(=22)(<21)) (and12’c’(fg)) (and)
=⇒ #t =⇒ #f =⇒ (fg) =⇒ #t
library syntax
library syntax
The ⟨test⟩ expressions are evaluated from left to right, and the value of the first expression that evaluates to a true value (see section 6.3.1) is returned. Any remaining ex- pressions are not evaluated. If all expressions evaluate to false values, the value of the last expression is returned. If there are no expressions then #f is returned.
(or(=22)(>21)) (or(=22)(<21)) (or #f #f #f)
(or (memq ’b ’(a b c))
(/30))
4.2.2. Binding constructs
=⇒ #t =⇒ #t =⇒ #f
=⇒ (bc)
(let ((x 2) (y 3))
(let* ((x 7)
(z (+ x y))) (*zx)))
The three binding constructs let, let*, and letrec give Scheme a block structure, like Algol 60. The syntax of the three constructs is identical, but they differ in the regions they establish for their variable bindings. In a let ex- pression, the initial values are computed before any of the variables become bound; in a let* expression, the bind- ings and evaluations are performed sequentially; while in a letrec expression, all the bindings are in effect while their
(letrec ⟨bindings⟩ ⟨body⟩)
Syntax: ⟨Bindings⟩ should have the form
(let ⟨bindings⟩ ⟨body⟩)
Syntax: ⟨Bindings⟩ should have the form
library syntax
=⇒ 70
library syntax
and ⟨body⟩ should be a sequence of one or more expres- sions. It is an error for a ⟨variable⟩ to appear more than once in the list of variables being bound.
Semantics: The ⟨variable⟩s are bound to fresh locations holding undefined values, the ⟨init⟩s are evaluated in the
((⟨variable1⟩ ⟨init1⟩) ...),
4. Expressions 11
12 Revised5 Scheme
resulting environment (in some unspecified order), each ⟨variable⟩ is assigned to the result of the corresponding ⟨init⟩, the ⟨body⟩ is evaluated in the resulting environment, and the value(s) of the last expression in ⟨body⟩ is(are) re- turned. Each binding of a ⟨variable⟩ has the entire letrec expression as its region, making it possible to define mutu- ally recursive procedures.
(letrec ((even?
(lambda (n)
(if (zero? n)
#t
(odd? (- n 1)))))
(odd?
(lambda (n)
(if (zero? n)
termination condition is met, the loop exits after evaluating the ⟨expression⟩s.
Do expressions are evaluated as follows: The ⟨init⟩ ex- pressions are evaluated (in some unspecified order), the ⟨variable⟩s are bound to fresh locations, the results of the ⟨init⟩ expressions are stored in the bindings of the ⟨variable⟩s, and then the iteration phase begins.
Each iteration begins by evaluating ⟨test⟩; if the result is false (see section 6.3.1), then the ⟨command⟩ expressions are evaluated in order for effect, the ⟨step⟩ expressions are evaluated in some unspecified order, the ⟨variable⟩s are bound to fresh locations, the results of the ⟨step⟩s are stored in the bindings of the ⟨variable⟩s, and the next iter- ation begins.
If ⟨test⟩ evaluates to a true value, then the ⟨expression⟩s are evaluated from left to right and the value(s) of the last ⟨expression⟩ is(are) returned. If no ⟨expression⟩s are present, then the value of the do expression is unspecified.
The region of the binding of a ⟨variable⟩ consists of the entire do expression except for the ⟨init⟩s. It is an error for a ⟨variable⟩ to appear more than once in the list of do variables.
A ⟨step⟩ may be omitted, in which case the effect is the same as if (⟨variable⟩ ⟨init⟩ ⟨variable⟩) had been written instead of (⟨variable⟩ ⟨init⟩).
(even? 88))
#f
(even? (- n 1))))))
=⇒ #t
One restriction on letrec is very important: it must be possible to evaluate each ⟨init⟩ without assigning or refer- ring to the value of any ⟨variable⟩. If this restriction is violated, then it is an error. The restriction is necessary because Scheme passes arguments by value rather than by name. In the most common uses of letrec, all the ⟨init⟩s are lambda expressions and the restriction is satisfied au- tomatically.
4.2.3. Sequencing
(begin ⟨expression1⟩
(do ((vec (make-vector 5))
(i 0 (+ i 1)))
((= i 5) vec)
(vector-set! vec i i))
(let ((x ’(1 3 5 7 9)))
(do ((x x (cdr x))
(sum 0 (+ sum (car x)))) ((null? x) sum))) =⇒ 25
(let ⟨variable⟩ ⟨bindings⟩ ⟨body⟩)
(define x 0)
(begin (set! x 5) (+x1))
library syntax
(begin (display
(display
"4 plus 1 equals ")
4.2.4. Iteration
(do ((⟨variable1 ⟩ ...)
⟨expression2⟩
. . . )
library syntax
The ⟨expression⟩s are evaluated sequentially from left to right, and the value(s) of the last ⟨expression⟩ is(are) re- turned. This expression type is used to sequence side ef- fects such as input and output.
(+ 4 1))) =⇒ unspecified and prints 4 plus 1 equals 5
⟨init1 ⟩ (⟨test⟩ ⟨expression⟩ ...)
⟨command⟩ ...)
⟨step1 ⟩)
library syntax
Do is an iteration construct. It specifies a set of variables to be bound, how they are to be initialized at the start, and how they are to be updated on each iteration. When a
=⇒ 6
“Named let” is a variant on the syntax of let which pro- vides a more general looping construct than do and may also be used to express recursions. It has the same syn- tax and semantics as ordinary let except that ⟨variable⟩ is bound within ⟨body⟩ to a procedure whose formal argu- ments are the bound variables and whose body is ⟨body⟩. Thus the execution of ⟨body⟩ may be repeated by invoking the procedure named by ⟨variable⟩.
(let loop ((numbers ’(3 -2 1 6 -5))
(nonneg ’())
(neg ’()))
(cond ((null? numbers) (list nonneg neg))
((>= (car numbers) 0)
(loop (cdr numbers)
(cons (car numbers) nonneg)
neg))
((< (car numbers) 0)
=⇒ #(0 1 2 3 4)
(loop (cdr numbers)
nonneg
(cons (car numbers) neg))))) =⇒ ((6 1 3) (-5 -2))
`(a `(b ,(+ 1 2) ,(foo ,(+ 1 3) d) e) f)
=⇒ (a`(b,(+12),(foo4d)e)f)
(let ((name1 ’x)
(name2 ’y))
`(a `(b ,,name1 ,’,name2 d) e)) =⇒ (a`(b,x,’yd)e)
(quasiquote (list (unquote (+ 1 2)) 4)) =⇒ (list 3 4)
’(quasiquote (list (unquote (+ 1 2)) 4)) =⇒ `(list ,(+ 1 2) 4)
i.e., (quasiquote (list (unquote (+ 1 2)) 4))
Unpredictable behavior can result if any of the symbols quasiquote, unquote, or unquote-splicing appear in po- sitions within a ⟨qq template⟩ otherwise than as described above.
4.3. Macros
Scheme programs can define and use new derived expres- sion types, called macros. Program-defined expression types have the syntax
(⟨keyword⟩ ⟨datum⟩ ...)
where ⟨keyword⟩ is an identifier that uniquely determines the expression type. This identifier is called the syntactic keyword, or simply keyword, of the macro. The number of the ⟨datum⟩s, and their syntax, depends on the expression type.
Each instance of a macro is called a use of the macro. The set of rules that specifies how a use of a macro is transcribed into a more primitive expression is called the transformer of the macro.
The macro definition facility consists of two parts:
• A set of expressions used to establish that certain iden- tifiers are macro keywords, associate them with macro transformers, and control the scope within which a macro is defined, and
• a pattern language for specifying macro transformers.
The syntactic keyword of a macro may shadow variable bindings, and local variable bindings may shadow keyword bindings. All macros defined using the pattern language are “hygienic” and “referentially transparent” and thus preserve Scheme’s lexical scoping [14, 15, 2, 7, 9]:
4.2.5. Delayed evaluation
(delay ⟨expression⟩)
two notations `⟨qq template⟩ and (quasiquote
The delay construct is used together with the proce- dure force to implement lazy evaluation or call by need. (delay ⟨expression⟩) returns an object called a promise which at some point in the future may be asked (by the force procedure) to evaluate ⟨expression⟩, and deliver the resulting value. The effect of ⟨expression⟩ returning multi- ple values is unspecified.
See the description of force (section 6.4) for a more com- plete description of delay.
4.2.6. Quasiquotation
(quasiquote ⟨qq template⟩) `⟨qq template⟩
syntax syntax
“Backquote” or “quasiquote” expressions are useful for constructing a list or vector structure when most but not all of the desired structure is known in advance. If no commas appear within the ⟨qq template⟩, the result of evaluating `⟨qq template⟩ is equivalent to the result of evaluating ’⟨qq template⟩. If a comma appears within the ⟨qq template⟩, however, the expression following the comma is evaluated (“unquoted”) and its result is inserted into the structure instead of the comma and the expres- sion. If a comma appears followed immediately by an at- sign (@), then the following expression must evaluate to a list; the opening and closing parentheses of the list are then “stripped away” and the elements of the list are in- serted in place of the comma at-sign expression sequence. A comma at-sign should only appear within a list or vector ⟨qq template⟩.
`(list ,(+ 1 2) 4) =⇒ (list 3 4) (let ((name ’a)) `(list ,name ’,name))
=⇒ (list a (quote a))
`(a ,(+ 1 2) ,@(map abs ’(4 -5 6)) b)
=⇒ (a3456b)
`(( foo ,(- 10 3)) ,@(cdr ’(c)) . ,(car ’(cons)))
=⇒ ((foo 7) . cons)
`#(10 5 ,(sqrt 4) ,@(map sqrt ’(16 9)) 8) =⇒ #(1052438)
Quasiquote forms may be nested. Substitutions are made only for unquoted components appearing at the same nest- ing level as the outermost backquote. The nesting level in- creases by one inside each successive quasiquotation, and decreases by one inside each unquotation.
library syntax
The
⟨qq template⟩) are identical in all respects. ,⟨expression⟩ is identical to (unquote ⟨expression⟩), and ,@⟨expression⟩ is identical to (unquote-splicing ⟨expression⟩). The ex- ternal syntax generated by write for two-element lists whose car is one of these symbols may vary between im- plementations.
4. Expressions 13
14
Revised5 Scheme
•
•
If a macro transformer inserts a binding for an identi- fier (variable or keyword), the identifier will in effect be renamed throughout its scope to avoid conflicts with other identifiers. Note that a define at top level may or may not introduce a binding; see section 5.2.
If a macro transformer inserts a free reference to an identifier, the reference refers to the binding that was visible where the transformer was specified, regardless of any local bindings that may surround the use of the macro.
of the letrec-syntax expression with macros whose key- words are the ⟨keyword⟩s, bound to the specified trans- formers. Each binding of a ⟨keyword⟩ has the ⟨bindings⟩ as well as the ⟨body⟩ within its region, so the transformers can transcribe expressions into uses of the macros intro- duced by the letrec-syntax expression.
(letrec-syntax
((my-or (syntax-rules ()
((my-or) #f)
((my-or e) e)
((my-or e1 e2 ...)
(let ((temp e1))
(if temp
temp
(my-or e2 ...)))))))
(let ((x #f)
(y 7)
(temp 8)
(let odd?)
(if even?))
(my-or x
(let temp)
(if y)
y))) =⇒ 7
4.3.2. Pattern language
A ⟨transformer spec⟩ has the following form: (syntax-rules ⟨literals⟩ ⟨syntax rule⟩ ...)
Syntax: ⟨Literals⟩ is a list of identifiers and each ⟨syntax rule⟩ should be of the form
(⟨pattern⟩ ⟨template⟩)
The ⟨pattern⟩ in a ⟨syntax rule⟩ is a list ⟨pattern⟩ that
begins with the keyword for the macro.
A ⟨pattern⟩ is either an identifier, a constant, or one of the following
(⟨pattern⟩ ...)
(⟨pattern⟩ ⟨pattern⟩ ... . ⟨pattern⟩) (⟨pattern⟩ ... ⟨pattern⟩ ⟨ellipsis⟩) #(⟨pattern⟩ ...)
#(⟨pattern⟩ ... ⟨pattern⟩ ⟨ellipsis⟩)
and a template is either an identifier, a constant, or one of the following
(⟨element⟩ ...)
(⟨element⟩ ⟨element⟩ ... . ⟨template⟩) #(⟨element⟩ ...)
where an ⟨element⟩ is a ⟨template⟩ optionally followed by an ⟨ellipsis⟩ and an ⟨ellipsis⟩ is the identifier “...” (which cannot be used as an identifier in either a template or a pattern).
Semantics: An instance of syntax-rules produces a new macro transformer by specifying a sequence of hygienic
4.3.1. Binding constructs for syntactic keywords
Let-syntax and letrec-syntax are analogous to let and letrec, but they bind syntactic keywords to macro trans- formers instead of binding variables to locations that con- tain values. Syntactic keywords may also be bound at top level; see section 5.3.
(let-syntax ⟨bindings⟩ ⟨body⟩) Syntax: ⟨Bindings⟩ should have the form
((⟨keyword⟩ ⟨transformer spec⟩) ...)
syntax
Each ⟨keyword⟩ is an identifier, each ⟨transformer spec⟩ is an instance of syntax-rules, and ⟨body⟩ should be a sequence of one or more expressions. It is an error for a ⟨keyword⟩ to appear more than once in the list of keywords being bound.
Semantics: The ⟨body⟩ is expanded in the syntactic envi- ronment obtained by extending the syntactic environment of the let-syntax expression with macros whose keywords are the ⟨keyword⟩s, bound to the specified transformers. Each binding of a ⟨keyword⟩ has ⟨body⟩ as its region.
(let-syntax ((when (syntax-rules ()
((when test stmt1 stmt2 ...)
(if test
(begin stmt1
stmt2 ...))))))
(let ((if #t))
(when if (set! if ’now))
if)) =⇒ now
(let ((x ’outer))
(let-syntax ((m (syntax-rules () ((m) x))))
(let ((x ’inner))
(m)))) =⇒ outer
(letrec-syntax ⟨bindings⟩ ⟨body⟩)
Syntax: Same as for let-syntax.
syntax
Semantics: The ⟨body⟩ is expanded in the syntactic envi-
ronment obtained by extending the syntactic environment
rewrite rules. A use of a macro whose keyword is associated with a transformer specified by syntax-rules is matched against the patterns contained in the ⟨syntax rule⟩s, be- ginning with the leftmost ⟨syntax rule⟩. When a match is found, the macro use is transcribed hygienically according to the template.
An identifier that appears in the pattern of a ⟨syntax rule⟩ is a pattern variable, unless it is the keyword that begins the pattern, is listed in ⟨literals⟩, or is the identifier “...”. Pattern variables match arbitrary input elements and are used to refer to elements of the input in the template. It is an error for the same pattern variable to appear more than once in a ⟨pattern⟩.
The keyword at the beginning of the pattern in a ⟨syntax rule⟩ is not involved in the matching and is not considered a pattern variable or literal identifier.
Rationale: The scope of the keyword is determined by the expression or syntax definition that binds it to the associated macro transformer. If the keyword were a pattern variable or literal identifier, then the template that follows the pattern would be within its scope regardless of whether the keyword were bound by let-syntax or by letrec-syntax.
Identifiers that appear in ⟨literals⟩ are interpreted as literal identifiers to be matched against corresponding subforms of the input. A subform in the input matches a literal identifier if and only if it is an identifier and either both its occurrence in the macro expression and its occurrence in the macro definition have the same lexical binding, or the two identifiers are equal and both have no lexical binding.
A subpattern followed by ... can match zero or more el- ements of the input. It is an error for ... to appear in ⟨literals⟩. Within a pattern the identifier ... must follow the last element of a nonempty sequence of subpatterns.
More formally, an input form F matches a pattern P if and only if:
• P is a non-literal identifier; or
• P is a literal identifier and F is an identifier with the
same binding; or
• P isalist(P1 ... Pn)andF isalistofnformsthat
match P1 through Pn, respectively; or
• P is an improper list (P1 P2 ... Pn . Pn+1) and F is a list or improper list of n or more forms that match P1 through Pn, respectively, and whose nth “cdr” matches Pn+1; or
• P is of the form (P1 ... Pn Pn+1 ⟨ellipsis⟩) where ⟨ellipsis⟩ is the identifier ... and F is a proper list of at least n forms, the first n of which match P1 through Pn, respectively, and each remaining element of F matches Pn+1; or
4. Expressions 15 •Pisavectoroftheform#(P1 ...Pn)andFisa
vector of n forms that match P1 through Pn; or
• P is of the form #(P1 . . . Pn Pn+1 ⟨ellipsis⟩) where ⟨ellipsis⟩ is the identifier ... and F is a vector of n or more forms the first n of which match P1 through Pn, respectively, and each remaining element of F matches Pn+1; or
• P isadatumandF isequaltoP inthesenseofthe equal? procedure.
It is an error to use a macro keyword, within the scope of its binding, in an expression that does not match any of the patterns.
When a macro use is transcribed according to the template of the matching ⟨syntax rule⟩, pattern variables that occur in the template are replaced by the subforms they match in the input. Pattern variables that occur in subpatterns followed by one or more instances of the identifier ... are allowed only in subtemplates that are followed by as many instances of .... They are replaced in the output by all of the subforms they match in the input, distributed as indicated. It is an error if the output cannot be built up as specified.
Identifiers that appear in the template but are not pattern variables or the identifier ... are inserted into the output as literal identifiers. If a literal identifier is inserted as a free identifier then it refers to the binding of that identifier within whose scope the instance of syntax-rules appears. If a literal identifier is inserted as a bound identifier then it is in effect renamed to prevent inadvertent captures of free identifiers.
As an example, if let and cond are defined as in section 7.3 then they are hygienic (as required) and the following is not an error.
(let ((=> #f))
(cond (#t => ’ok)))
=⇒ ok
The macro transformer for cond recognizes => as a local variable, and hence an expression, and not as the top-level identifier =>, which the macro transformer treats as a syn- tactic keyword. Thus the example expands into
(let ((=> #f))
(if #t (begin => ’ok)))
instead of
(let ((=> #f))
(let ((temp #t))
(if temp (’ok temp))))
which would result in an invalid procedure call.
16 Revised5 Scheme
5. Program structure
5.1. Programs
A Scheme program consists of a sequence of expressions, definitions, and syntax definitions. Expressions are de- scribed in chapter 4; definitions and syntax definitions are the subject of the rest of the present chapter.
Programs are typically stored in files or entered inter- actively to a running Scheme system, although other paradigms are possible; questions of user interface lie out- side the scope of this report. (Indeed, Scheme would still be useful as a notation for expressing computational methods even in the absence of a mechanical implementation.)
Definitions and syntax definitions occurring at the top level of a program can be interpreted declaratively. They cause bindings to be created in the top level environment or mod- ify the value of existing top-level bindings. Expressions occurring at the top level of a program are interpreted im- peratively; they are executed in order when the program is invoked or loaded, and typically perform some kind of initialization.
At the top level of a program (begin ⟨form1⟩ …) is equivalent to the sequence of expressions, definitions, and syntax definitions that form the body of the begin.
5.2. Definitions
Definitions are valid in some, but not all, contexts where expressions are allowed. They are valid only at the top level of a ⟨program⟩ and at the beginning of a ⟨body⟩.
A definition should have one of the following forms: • (define ⟨variable⟩ ⟨expression⟩)
• (define (⟨variable⟩ ⟨formals⟩) ⟨body⟩)
⟨Formals⟩ should be either a sequence of zero or more variables, or a sequence of one or more variables fol- lowed by a space-delimited period and another vari- able (as in a lambda expression). This form is equiv- alent to
(define ⟨variable⟩
(lambda (⟨formals⟩) ⟨body⟩)).
• (define (⟨variable⟩ . ⟨formal⟩) ⟨body⟩)
⟨Formal⟩ should be a single variable. This form is
equivalent to
(define ⟨variable⟩
(lambda ⟨formal⟩ ⟨body⟩)).
5.2.1. Top level definitions
At the top level of a program, a definition
(define ⟨variable⟩ ⟨expression⟩)
has essentially the same effect as the assignment expres- sion
(set! ⟨variable⟩ ⟨expression⟩)
if ⟨variable⟩ is bound. If ⟨variable⟩ is not bound, however, then the definition will bind ⟨variable⟩ to a new location before performing the assignment, whereas it would be an error to perform a set! on an unbound variable.
(define add3
(lambda (x) (+ x 3)))
(add33)
(define first car) (first ’(1 2))
=⇒ 6 =⇒ 1
Some implementations of Scheme use an initial environ- ment in which all possible variables are bound to locations, most of which contain undefined values. Top level defini- tions in such an implementation are truly equivalent to assignments.
5.2.2. Internal definitions
Definitions may occur at the beginning of a ⟨body⟩ (that is, the body of a lambda, let, let*, letrec, let-syntax, or letrec-syntax expression or that of a definition of an appropriate form). Such definitions are known as internal definitions as opposed to the top level definitions described above. The variable defined by an internal definition is local to the ⟨body⟩. That is, ⟨variable⟩ is bound rather than assigned, and the region of the binding is the entire ⟨body⟩. For example,
(let ((x 5))
(define foo (lambda (y) (bar x y))) (define bar (lambda (a b) (+ (* a b) a))) (foo (+ x 3))) =⇒ 45
A ⟨body⟩ containing internal definitions can always be con- verted into a completely equivalent letrec expression. For example, the let expression in the above example is equiv- alent to
(let ((x 5))
(letrec ((foo (lambda (y) (bar x y)))
(bar (lambda (a b) (+ (* a b) a))))
(foo (+ x 3))))
Just as for the equivalent letrec expression, it must be possible to evaluate each ⟨expression⟩ of every internal def- inition in a ⟨body⟩ without assigning or referring to the value of any ⟨variable⟩ being defined.
Wherever an internal definition may occur (begin ⟨definition1⟩ . . . ) is equivalent to the sequence of defini- tions that form the body of the begin.
5.3. Syntax definitions
Syntax definitions are valid only at the top level of a ⟨program⟩. They have the following form:
(define-syntax ⟨keyword⟩ ⟨transformer spec⟩)
⟨Keyword⟩ is an identifier, and the ⟨transformer spec⟩ should be an instance of syntax-rules. The top-level syn- tactic environment is extended by binding the ⟨keyword⟩ to the specified transformer.
There is no define-syntax analogue of internal defini- tions.
Although macros may expand into definitions and syntax definitions in any context that permits them, it is an error for a definition or syntax definition to shadow a syntactic keyword whose meaning is needed to determine whether some form in the group of forms that contains the shad- owing definition is in fact a definition, or, for internal def- initions, is needed to determine the boundary between the group and the expressions that follow the group. For ex- ample, the following are errors:
top-level binding that has not been introduced by a defini- tion has an unspecified effect on the behavior of the built-in procedures.
6.1. Equivalence predicates
A predicate is a procedure that always returns a boolean value (#t or #f). An equivalence predicate is the compu- tational analogue of a mathematical equivalence relation (it is symmetric, reflexive, and transitive). Of the equiva- lence predicates described in this section, eq? is the finest or most discriminating, and equal? is the coarsest. Eqv? is slightly less discriminating than eq?.
6.
(define define 3)
(begin (define begin list))
(let-syntax
((foo (syntax-rules ()
((foo (proc args …) body …)
(define proc
(lambda (args …)
body …))))))
(let ((x 3))
(foo (plus x y) (+ x y))
(define foo x)
(plus foo x)))
Standard procedures
The eqv? procedure defines a useful equivalence relation on objects. Briefly, it returns #t if obj1 and obj2 should normally be regarded as the same object. This relation is left slightly open to interpretation, but the following par- tial specification of eqv? holds for all implementations of Scheme.
The eqv? procedure returns #t if:
This chapter describes Scheme’s built-in procedures. The initial (or “top level”) Scheme environment starts out with a number of variables bound to locations containing useful values, most of which are primitive procedures that ma- nipulate data. For example, the variable abs is bound to (a location initially containing) a procedure of one argu- ment that computes the absolute value of a number, and the variable + is bound to a procedure that computes sums. Built-in procedures that can easily be written in terms of other built-in procedures are identified as “library proce- dures”.
A program may use a top-level definition to bind any vari- able. It may subsequently alter any such binding by an assignment (see 4.1.6). These operations do not modify the behavior of Scheme’s built-in procedures. Altering any
equal (section 4.1.4).
The eqv? procedure returns #f if:
• obj1 and obj2 are of different types (section 3.2).
(eqv? obj1 obj2)
procedure
• •
•
•
• •
•
obj1 and obj2 are both #t or both #f. obj1 and obj2 are both symbols and
(string=? (symbol->string obj1)
(symbol->string obj2))
6. Standard procedures 17
Note:
obj1 and obj2 are both numbers, are numerically equal (see =, section 6.2), and are either both exact or both inexact.
obj1 and obj2 are both characters and are the same character according to the char=? procedure (sec- tion 6.3.4).
both obj1 and obj2 are the empty list.
obj1 and obj2 are pairs, vectors, or strings that denote
the same locations in the store (section 3.4).
obj1 and obj2 are procedures whose location tags are
This assumes that neither obj1 nor obj2 is an “un- interned symbol” as alluded to in section 6.3.3. This re- port does not presume to specify the behavior of eqv? on implementation-dependent extensions.
=⇒ #t
18 Revised5 Scheme
• •
• • • • • •
one of obj1 and obj2 is #t but the other is #f. obj1 and obj2 are symbols but
(string=? (symbol->string obj1) (symbol->string obj2))
=⇒ #f
one of obj1 and obj2 is an exact number but the other
is an inexact number.
obj1 and obj2 are numbers for which the = procedure
returns #f.
obj1 and obj2 are characters for which the char=? pro-
cedure returns #f.
one of obj1 and obj2 is the empty list but the other is
not.
obj1 and obj2 are pairs, vectors, or strings that denote distinct locations.
obj1 and obj2 are procedures that would behave differ- ently (return different value(s) or have different side effects) for some arguments.
=⇒ #f =⇒ #f
(define gen-counter
(lambda ()
(let ((n 0))
(lambda () (set! n (+ n 1)) n))))
(let ((g (gen-counter)))
(eqv? g g)) =⇒ #t
(eqv? (gen-counter) (gen-counter)) =⇒ #f
(define gen-loser
(lambda ()
(let ((n 0))
(lambda () (set! n (+ n 1)) 27))))
(let ((g (gen-loser)))
(eqv? g g)) =⇒ #t
(eqv? (gen-loser) (gen-loser))
=⇒ unspecified
(letrec ((f (lambda () (if (eqv? f g) ’both ’f)))
(g (lambda () (if (eqv? f g) ’both ’g))))
(eqv? f g))
=⇒ unspecified
(letrec ((f (lambda () (if (eqv? f g) ’f ’both)))
(g (lambda () (if (eqv? f g) ’g ’both))))
(eqv? f g))
=⇒ #f
Since it is an error to modify constant objects (those re- turned by literal expressions), implementations are per- mitted, though not required, to share structure between constants where appropriate. Thus the value of eqv? on constants is sometimes implementation-dependent.
(eqv? ’a ’a)
(eqv? ’a ’b)
(eqv? 2 2)
(eqv? ’() ’())
(eqv? 100000000 100000000)
(eqv? (cons 1 2) (cons 1 2))=⇒ #f (eqv? (lambda () 1)
(eqv? ’(a) ’(a))
(eqv? “a” “a”)
(eqv? ’(b) (cdr ’(a b)))
(let ((x ’(a)))
(eqv? x x))
=⇒ unspecified =⇒ unspecified =⇒ unspecified
=⇒ #t
(lambda () 2))
(eqv? #f ’nil)
(let ((p (lambda (x) x)))
(eqv? p p))
=⇒
#t
Rationale: The above definition of eqv? allows implementa- tions latitude in their treatment of procedures and literals: im- plementations are free either to detect or to fail to detect that two procedures or two literals are equivalent to each other, and can decide whether or not to merge representations of equivalent objects by using the same pointer or bit pattern to represent both.
(eq? obj1 obj2) procedure Eq? is similar to eqv? except that in some cases it is capable
of discerning distinctions finer than those detectable by eqv?.
Eq? and eqv? are guaranteed to have the same behavior on symbols, booleans, the empty list, pairs, procedures, and non-empty strings and vectors. Eq?’s behavior on numbers and characters is implementation-dependent, but it will al- ways return either true or false, and will return true only when eqv? would also return true. Eq? may also behave differently from eqv? on empty vectors and empty strings.
The following examples illustrate cases in which the above rules do not fully specify the behavior of eqv?. All that can be said about such cases is that the value returned by eqv? must be a boolean.
(eqv? “” “”)
(eqv? ’#() ’#())
(eqv? (lambda (x) x)
(lambda (x) x))
(eqv? (lambda (x) x)
(lambda (y) y))
=⇒ unspecified =⇒ unspecified
=⇒ unspecified
=⇒ unspecified
The next set of examples shows the use of eqv? with pro- cedures that have local state. Gen-counter must return a distinct procedure every time, since each procedure has its own internal counter. Gen-loser, however, returns equiv- alent procedures each time, since the local state does not affect the value or side effects of the procedures.
=⇒ #t =⇒ #f =⇒ #t =⇒ #t =⇒ #t
(eq? ’a ’a)
(eq? ’(a) ’(a))
(eq? (list ’a) (list ’a))
(eq? “a” “a”)
(eq? “” “”)
(eq? ’() ’())
(eq? 2 2)
(eq? #\A #\A)
(eq? car car)
(let ((n (+ 2 3)))
(eq? n n))
(let ((x ’(a)))
(eq? x x))
(let ((x ’#()))
(eq? x x))
(let ((p (lambda (x) x)))
(eq? p p))
=⇒ #t
=⇒ unspecified =⇒ #f
=⇒ unspecified =⇒ unspecified =⇒ #t
=⇒ unspecified =⇒ unspecified =⇒ #t
=⇒ unspecified
=⇒ #t
=⇒ #t
=⇒ #t
It is important to distinguish between the mathemati- cal numbers, the Scheme numbers that attempt to model them, the machine representations used to implement the Scheme numbers, and notations used to write numbers. This report uses the types number, complex, real, rational, and integer to refer to both mathematical numbers and Scheme numbers. Machine representations such as fixed point and floating point are referred to by names such as fixnum and flonum.
6.2.1. Numerical types
Mathematically, numbers may be arranged into a tower of subtypes in which each level is a subset of the level above it:
number complex real rational integer
For example, 3 is an integer. Therefore 3 is also a rational, a real, and a complex. The same is true of the Scheme numbers that model 3. For Scheme numbers, these types are defined by the predicates number?, complex?, real?, rational?, and integer?.
There is no simple relationship between a number’s type and its representation inside a computer. Although most implementations of Scheme will offer at least two different representations of 3, these different representations denote the same integer.
Scheme’s numerical operations treat numbers as abstract data, as independent of their representation as possible. Although an implementation of Scheme may use fixnum, flonum, and perhaps other representations for numbers, this should not be apparent to a casual programmer writing simple programs.
It is necessary, however, to distinguish between numbers that are represented exactly and those that may not be. For example, indexes into data structures must be known exactly, as must some polynomial coefficients in a symbolic algebra system. On the other hand, the results of measure- ments are inherently inexact, and irrational numbers may be approximated by rational and therefore inexact approx- imations. In order to catch uses of inexact numbers where exact numbers are required, Scheme explicitly distinguishes exact from inexact numbers. This distinction is orthogonal to the dimension of type.
6.2.2. Exactness
Scheme numbers are either exact or inexact. A number is exact if it was written as an exact constant or was derived from exact numbers using only exact operations. A number
Rationale: It will usually be possible to implement eq? much more efficiently than eqv?, for example, as a simple pointer com- parison instead of as some more complicated operation. One reason is that it may not be possible to compute eqv? of two numbers in constant time, whereas eq? implemented as pointer comparison will always finish in constant time. Eq? may be used like eqv? in applications using procedures to implement objects with state since it obeys the same constraints as eqv?.
(equal? obj1 obj2) library procedure
Equal? recursively compares the contents of pairs, vectors, and strings, applying eqv? on other objects such as num- bers and symbols. A rule of thumb is that objects are generally equal? if they print the same. Equal? may fail to terminate if its arguments are circular data structures.
(equal? ’a ’a)
(equal? ’(a) ’(a))
(equal? ’(a (b) c)
’(a (b) c))
(equal? “abc” “abc”)
(equal? 22)
(equal? (make-vector 5 ’a)
(make-vector 5 ’a))
(equal? (lambda (x) x)
(lambda (y) y))
6.2. Numbers
=⇒ #t =⇒ #t
=⇒ #t =⇒ #t =⇒ #t
=⇒ #t
=⇒ unspecified
Numerical computation has traditionally been neglected by the Lisp community. Until Common Lisp there was no carefully thought out strategy for organizing numerical computation, and with the exception of the MacLisp sys- tem [20] little effort was made to execute numerical code efficiently. This report recognizes the excellent work of the Common Lisp committee and accepts many of their rec- ommendations. In some ways this report simplifies and generalizes their proposals in a manner consistent with the purposes of Scheme.
6. Standard procedures 19
20 Revised5 Scheme
is inexact if it was written as an inexact constant, if it was derived using inexact ingredients, or if it was derived using inexact operations. Thus inexactness is a contagious property of a number.
If two implementations produce exact results for a com- putation that did not involve inexact intermediate results, the two ultimate results will be mathematically equivalent. This is generally not true of computations involving inex- act numbers since approximate methods such as floating point arithmetic may be used, but it is the duty of each implementation to make the result as close as practical to the mathematically ideal result.
Rational operations such as + should always produce ex- act results when given exact arguments. If the operation is unable to produce an exact result, then it may either report the violation of an implementation restriction or it may silently coerce its result to an inexact value. See sec- tion 6.2.3.
With the exception of inexact->exact, the operations de- scribed in this section must generally return inexact results when given any inexact arguments. An operation may, however, return an exact result if it can prove that the value of the result is unaffected by the inexactness of its arguments. For example, multiplication of any number by an exact zero may produce an exact zero result, even if the other argument is inexact.
6.2.3. Implementation restrictions
Implementations of Scheme are not required to implement the whole tower of subtypes given in section 6.2.1, but they must implement a coherent subset consistent with both the purposes of the implementation and the spirit of the Scheme language. For example, an implementation in which all numbers are real may still be quite useful.
Implementations may also support only a limited range of numbers of any type, subject to the requirements of this section. The supported range for exact numbers of any type may be different from the supported range for inex- act numbers of that type. For example, an implementation that uses flonums to represent all its inexact real numbers may support a practically unbounded range of exact inte- gers and rationals while limiting the range of inexact reals (and therefore the range of inexact integers and rationals) to the dynamic range of the flonum format. Furthermore the gaps between the representable inexact integers and ra- tionals are likely to be very large in such an implementation as the limits of this range are approached.
An implementation of Scheme must support exact integers throughout the range of numbers that may be used for indexes of lists, vectors, and strings or that may result from computing the length of a list, vector, or string. The length, vector-length, and string-length procedures
must return an exact integer, and it is an error to use anything but an exact integer as an index. Furthermore any integer constant within the index range, if expressed by an exact integer syntax, will indeed be read as an exact integer, regardless of any implementation restrictions that may apply outside this range. Finally, the procedures listed below will always return an exact integer result provided all their arguments are exact integers and the mathematically expected result is representable as an exact integer within the implementation:
+-*
quotient
max
numerator
lcm
truncate
expt
remainder
min
denominator
floor
round
modulo
abs
gcd
ceiling
rationalize
Implementations are encouraged, but not required, to sup- port exact integers and exact rationals of practically unlim- ited size and precision, and to implement the above proce- dures and the / procedure in such a way that they always return exact results when given exact arguments. If one of these procedures is unable to deliver an exact result when given exact arguments, then it may either report a vio- lation of an implementation restriction or it may silently coerce its result to an inexact number. Such a coercion may cause an error later.
An implementation may use floating point and other ap- proximate representation strategies for inexact numbers. This report recommends, but does not require, that the IEEE 32-bit and 64-bit floating point standards be followed by implementations that use flonum representations, and that implementations using other representations should match or exceed the precision achievable using these float- ing point standards [12].
In particular, implementations that use flonum represen- tations must follow these rules: A flonum result must be represented with at least as much precision as is used to express any of the inexact arguments to that operation. It is desirable (but not required) for potentially inexact oper- ations such as sqrt, when applied to exact arguments, to produce exact answers whenever possible (for example the square root of an exact 4 ought to be an exact 2). If, how- ever, an exact number is operated upon so as to produce an inexact result (as by sqrt), and if the result is represented as a flonum, then the most precise flonum format available must be used; but if the result is represented in some other way then the representation must have at least as much precision as the most precise flonum format available.
Although Scheme allows a variety of written notations for numbers, any particular implementation may support only some of them. For example, an implementation in which all numbers are real need not support the rectangular and
polar notations for complex numbers. If an implementa- tion encounters an exact numerical constant that it cannot represent as an exact number, then it may either report a violation of an implementation restriction or it may silently represent the constant by an inexact number.
6.2.4. Syntax of numerical constants
The syntax of the written representations for numbers is described formally in section 7.1.1. Note that case is not significant in numerical constants.
A number may be written in binary, octal, decimal, or hex- adecimal by the use of a radix prefix. The radix prefixes are #b (binary), #o (octal), #d (decimal), and #x (hexadec- imal). With no radix prefix, a number is assumed to be expressed in decimal.
A numerical constant may be specified to be either exact or inexact by a prefix. The prefixes are #e for exact, and #i for inexact. An exactness prefix may appear before or after any radix prefix that is used. If the written representation of a number has no exactness prefix, the constant may be either inexact or exact. It is inexact if it contains a decimal point, an exponent, or a “#” character in the place of a digit, otherwise it is exact.
In systems with inexact numbers of varying precisions it may be useful to specify the precision of a constant. For this purpose, numerical constants may be written with an exponent marker that indicates the desired precision of the inexact representation. The letters s, f, d, and l specify the use of short, single, double, and long precision, respec- tively. (When fewer than four internal inexact represen- tations exist, the four size specifications are mapped onto those available. For example, an implementation with two internal representations may map short and single together and long and double together.) In addition, the exponent marker e specifies the default precision for the implemen- tation. The default precision has at least as much precision as double, but implementations may wish to allow this de- fault to be set by the user.
3.14159265358979F0
Round to single — 3.141593
0.6L0
Extend to long — .600000000000000 6.2.5. Numerical operations
The reader is referred to section 1.3.3 for a summary of the naming conventions used to specify restrictions on the types of arguments to numerical routines. The examples used in this section assume that any numerical constant written using an exact notation is indeed represented as an exact number. Some examples also assume that certain numerical constants written using an inexact notation can
be represented without loss of accuracy; the inexact con- stants were chosen so that this is likely to be true in imple- mentations that use flonums to represent inexact numbers.
(number? obj ) (complex? obj ) (real? obj ) (rational? obj ) (integer? obj )
procedure procedure procedure procedure procedure
These numerical type predicates can be applied to any kind of argument, including non-numbers. They return #t if the object is of the named type, and otherwise they return #f. In general, if a type predicate is true of a number then all higher type predicates are also true of that number. Consequently, if a type predicate is false of a number, then all lower type predicates are also false of that number.
If z is an inexact complex number, then (real? z) is true if and only if (zero? (imag-part z)) is true. If x is an inexact real number, then (integer? x) is true if and only if (= x (round x)).
(complex?
(complex?
(real? 3)
(real? -2.5+0.0i)
(real? #e1e10)
(rational? 6/10)
(rational? 6/3)
(integer? 3+0i)
(integer? 3.0)
(integer? 8/4)
=⇒ #t =⇒ #t =⇒ #t =⇒ #t =⇒ #t =⇒ #t =⇒ #t =⇒ #t =⇒ #t =⇒ #t
3+4i) 3)
Note: The behavior of these type predicates on inexact num- bers is unreliable, since any inaccuracy may affect the result.
Note: In many implementations the rational? procedure will be the same as real?, and the complex? procedure will be the same as number?, but unusual implementations may be able to represent some irrational numbers exactly or may extend the number system to support some kind of non-complex numbers.
(exact? z) (inexact? z)
procedure procedure
These numerical predicates provide tests for the exactness of a quantity. For any Scheme number, precisely one of these predicates is true.
(=z1z2z3…) (
procedure procedure procedure procedure procedure
These procedures return #t if their arguments are (respec- tively): equal, monotonically increasing, monotonically de- creasing, monotonically nondecreasing, or monotonically nonincreasing.
6. Standard procedures 21
22 Revised5 Scheme
These predicates are required to be transitive.
Note: The traditional implementations of these predicates in Lisp-like languages are not transitive.
Note: While it is not an error to compare inexact numbers using these predicates, the results may be unreliable because a small inaccuracy may affect the result; this is especially true of = and zero?. When in doubt, consult a numerical analyst.
=⇒ -1 =⇒ -6 =⇒ -3 =⇒ 3/20 =⇒ 1/3
(abs x)
Abs returns the absolute value of its argument.
(zero? z) (positive? x) (negative? x) (odd? n) (even? n)
library procedure library procedure library procedure library procedure library procedure
(abs-7)
(quotient n1 n2) (remainder n1 n2) (modulo n1 n2)
=⇒ 7
These numerical predicates test a number for a particular property, returning #t or #f. See note above.
(max x1 x2 . . . ) library procedure (min x1 x2 . . . ) library procedure
These procedures return the maximum or minimum of their arguments.
procedure procedure procedure
(max 3 4)
(max 3.9 4)
=⇒ 4 ; exact =⇒ 4.0 ; inexact
(quotient n1 n2 ) (remainder n1 n2 ) (modulo n1 n2 )
If n1/n2 is not an integer: (quotient n1 n2 )
(remainder n1 n2 ) (modulo n1 n2 )
=⇒ n1 /n2 =⇒ 0
=⇒ 0
=⇒ nq =⇒ nr =⇒ nm
Note: If any argument is inexact, then the result will also be inexact (unless the procedure can prove that the inaccuracy is not large enough to affect the result, which is possible only in unusual implementations). If min or max is used to compare numbers of mixed exactness, and the numerical value of the result cannot be represented as an inexact number without loss of accuracy, then the procedure may report a violation of an implementation restriction.
where nq is n1/n2 rounded towards zero, 0 < |nr| < |n2|, 0 < |nm| < |n2|, nr and nm differ from n1 by a multiple of n2, nr has the same sign as n1, and nm has the same sign as n2.
From this we can conclude that for integers n1 and n2 with n2 not equal to 0,
(= n1 (+ (* n2 (quotient n1 n2)) (remainder n1 n2 )))
=⇒ #t
provided all numbers involved in that computation are ex-
(+ z1 (* z1
. . . ) . . . )
procedure procedure
These procedures return the sum or product of their argu- ments.
(- z1
(- z)
(- z1
(/ z1
(/ z)
(/ z1
z2)
z2 . . . )
z2)
z2 . . . )
procedure procedure procedure procedure procedure procedure
(modulo 13 4)
(remainder 13 4)
(modulo -13 4)
=⇒ 1 =⇒ 1
=⇒ 3 =⇒ -1
=⇒ -3 =⇒ 1
=⇒ -1 =⇒ -1
=⇒ -1.0 ; inexact
(+ 3 4)
(+ 3)
(+)
(* 4)
(*)
=⇒ 7 =⇒ 3 =⇒ 0 =⇒ 4 =⇒ 1
optional
optional
act.
With two or more arguments, these procedures return the difference or quotient of their arguments, associating to the left. With one argument, however, they return the additive or multiplicative inverse of their argument.
(modulo -13 -4)
(remainder -13 -4)
(remainder -13 -4.0)
(- 3 4)
(- 3 4 5)
(- 3)
(/ 3 4 5)
(/ 3)
These procedures implement number-theoretic (integer) di- vision. n2 should be non-zero. All three procedures return integers. If n1 /n2 is an integer:
(remainder
(modulo 13
(remainder
-13 4)
-4)
13 -4)
library procedure
(gcd n1 . . . ) library procedure (lcm n1 . . . ) library procedure
These procedures return the greatest common divisor or least common multiple of their arguments. The result is always non-negative.
(rationalize x y )
6. Standard procedures 23 library procedure
(gcd 32 -36)
(gcd)
(lcm 32 -36)
(lcm 32.0 -36)
(lcm)
(numerator q) (denominator q)
=⇒ 4
=⇒ 0
=⇒ 288
=⇒ 288.0 ; inexact =⇒ 1
procedure procedure
Rationalize returns the simplest rational number differ- ing from x by no more than y. A rational number r1 is simpler than another rational number r2 if r1 = p1/q1 and r2 = p2/q2 (in lowest terms) and |p1| ≤ |p2| and |q1| ≤ |q2|. Thus 3/5 is simpler than 4/7. Although not all rationals are comparable in this ordering (consider 2/7 and 3/5) any interval contains a rational number that is simpler than ev- ery other rational number in that interval (the simpler 2/5 lies between 2/7 and 3/5). Note that 0 = 0/1 is the sim- plest rational of all.
(rationalize .3 1/10)
(exp z) (log z) (sin z) (cos z) (tan z) (asin z) (acos z) (atan z) (atan y x)
These procedures return the numerator or denominator of their argument; the result is computed as if the argument was represented as a fraction in lowest terms. The denom- inator is always positive. The denominator of 0 is defined to be 1.
; exact =⇒ #i1/3 ; inexact
(numerator (/ 6 4))
(denominator (/ 6 4))
(denominator
=⇒ 3 =⇒ 2
procedure procedure procedure procedure procedure procedure procedure procedure procedure
(exact->inexact (/ 6 4))) =⇒ 2.0
(floor x) (ceiling x) (truncate x) (round x)
procedure procedure procedure procedure
These procedures are part of every implementation that supports general real numbers; they compute the usual transcendental functions. Log computes the natural log- arithm of z (not the base ten logarithm). Asin, acos, and atan compute arcsine (sin−1), arccosine (cos−1), and arctangent (tan−1), respectively. The two-argument vari- ant of atan computes (angle (make-rectangular x y)) (see below), even in implementations that don’t support general complex numbers.
In general, the mathematical functions log, arcsine, arc- cosine, and arctangent are multiply defined. The value of log z is defined to be the one whose imaginary part lies in the range from −π (exclusive) to π (inclusive). log 0 is un- defined. With log defined this way, the values of sin−1 z, cos−1 z, and tan−1 z are according to the following for- mulæ:
sin−1 z = −i log(iz + 1 − z2) cos−1 z = π/2 − sin−1 z
tan−1 z = (log(1 + iz) − log(1 − iz))/(2i)
The above specification follows [27], which in turn cites [19]; refer to these sources for more detailed discussion of branch cuts, boundary conditions, and implementation of these functions. When it is possible these procedures produce a real result from a real argument.
These procedures return integers. Floor returns the largest integer not larger than x. Ceiling returns the smallest integer not smaller than x. Truncate returns the integer closest to x whose absolute value is not larger than the absolute value of x. Round returns the closest inte- ger to x, rounding to even when x is halfway between two integers.
Rationale: Round rounds to even for consistency with the de- fault rounding mode specified by the IEEE floating point stan- dard.
Note: If the argument to one of these procedures is inexact, then the result will also be inexact. If an exact value is needed, the result should be passed to the inexact->exact procedure.
(rationalize
(inexact->exact .3) 1/10) =⇒ 1/3
(floor -4.3)
(ceiling -4.3)
(truncate -4.3)
(round -4.3)
(floor 3.5)
(ceiling 3.5)
(truncate 3.5)
(round 3.5)
(round 7/2)
(round 7)
=⇒ -5.0 =⇒ -4.0 =⇒ -4.0 =⇒ -4.0
=⇒ 3.0
=⇒ 4.0
=⇒ 3.0
=⇒ 4.0 ; inexact
=⇒ 4 ; exact =⇒ 7
24 Revised5 Scheme
(sqrt z) procedure
Returns the principal square root of z. The result will have either positive real part, or zero real part and non-negative imaginary part.
6.2.6. Numerical input and output
(number->string z) (number->string z radix)
procedure procedure
(expt z1 z2)
Returns z1 raised to the power z2. For z1 ̸= 0
z1z2 =ez2logz1 0z is1ifz=0and0otherwise.
procedure
procedure procedure procedure procedure procedure procedure
Radix must be an exact integer, either 2, 8, 10, or 16. If omitted, radix defaults to 10. The procedure number-> string takes a number and a radix and returns as a string an external representation of the given number in the given radix such that
(let ((number number) (radix radix))
(eqv? number
(string->number (number->string number
radix)
radix)))
is true. It is an error if no possible result makes this ex- pression true.
If z is inexact, the radix is 10, and the above expression can be satisfied by a result that contains a decimal point, then the result contains a decimal point and is expressed using the minimum number of digits (exclusive of exponent and trailing zeroes) needed to make the above expression true [3, 5]; otherwise the format of the result is unspecified.
The result returned by number->string never contains an explicit radix prefix.
Note: The error case can occur only when z is not a complex number or is a complex number with a non-rational real or imaginary part.
Rationale: If z is an inexact number represented using flonums, and the radix is 10, then the above expression is normally satis- fied by a result containing a decimal point. The unspecified case allows for infinities, NaNs, and non-flonum representations.
(make-rectangular x1
x2 )
(make-polar x3 (real-part z) (imag-part z) (magnitude z) (angle z)
x4 )
These procedures are part of every implementation that supports general complex numbers. Suppose x1, x2, x3, and x4 are real numbers and z is a complex number such that
Then
(make-rectangular x1
x2 )
=⇒ z
=⇒ z
=⇒ x1 =⇒ x2 =⇒ |x3 | =⇒ xangle
(make-polar x3 (real-part z) (imag-part z) (magnitude z) (angle z)
x4 )
z=x +x i=x ·eix 1234
where −π < xangle ≤ π with xangle = x4 + 2πn for some integer n.
Rationale: Magnitude is the same as abs for a real argu- ment, but abs must be present in all implementations, whereas magnitude need only be present in implementations that sup- port general complex numbers.
(string->number string) (string->number string radix)
procedure procedure
(exact->inexact z) (inexact->exact z)
procedure procedure
Returns a number of the maximally precise representation expressed by the given string. Radix must be an exact integer, either 2, 8, 10, or 16. If supplied, radix is a default radix that may be overridden by an explicit radix prefix in string (e.g. “#o177”). If radix is not supplied, then the default radix is 10. If string is not a syntactically valid notation for a number, then string->number returns #f.
(string->number “100”) =⇒ 100 (string->number “100” 16) =⇒ 256 (string->number “1e2”) =⇒ 100.0 (string->number “15##”) =⇒ 1500.0
Note: The domain of string->number may be restricted by implementations in the following ways. String->number is per- mitted to return #f whenever string contains an explicit radix prefix. If all numbers supported by an implementation are real,
Exact->inexact returns an inexact representation of z. The value returned is the inexact number that is numeri- cally closest to the argument. If an exact argument has no reasonably close inexact equivalent, then a violation of an implementation restriction may be reported.
Inexact->exact returns an exact representation of z. The value returned is the exact number that is numerically clos- est to the argument. If an inexact argument has no rea- sonably close exact equivalent, then a violation of an im- plementation restriction may be reported.
These procedures implement the natural one-to-one corre- spondence between exact and inexact integers throughout an implementation-dependent range. See section 6.2.3.
then string->number is permitted to return #f whenever string uses the polar or rectangular notations for complex numbers. If all numbers are integers, then string->number may return #f whenever the fractional notation is used. If all numbers are exact, then string->number may return #f whenever an ex- ponent marker or explicit exactness prefix is used, or if a # appears in place of a digit. If all inexact numbers are integers, then string->number may return #f whenever a decimal point is used.
6.3. Other data types
This section describes operations on some of Scheme’s non- numeric data types: booleans, pairs, lists, symbols, char- acters, strings and vectors.
6.3.1. Booleans
The standard boolean objects for true and false are written as #t and #f. What really matters, though, are the objects that the Scheme conditional expressions (if, cond, and, or, do) treat as true or false. The phrase “a true value” (or sometimes just “true”) means any object treated as true by the conditional expressions, and the phrase “a false value” (or “false”) means any object treated as false by the conditional expressions.
Of all the standard Scheme values, only #f counts as false in conditional expressions. Except for #f, all standard Scheme values, including #t, pairs, the empty list, sym- bols, numbers, strings, vectors, and procedures, count as true.
Note: Programmers accustomed to other dialects of Lisp should be aware that Scheme distinguishes both #f and the empty list from the symbol nil.
Boolean constants evaluate to themselves, so they do not need to be quoted in programs.
(boolean? obj )
Boolean? returns #t if obj is either #t or #f and returns
=⇒ #t =⇒ #f =⇒ #f
(not obj)
Not returns #t if obj is false, and returns #f otherwise.
(not #t) =⇒#f (not 3) =⇒#f (not (list 3)) =⇒#f (not #f) =⇒#t (not ’()) =⇒#f (not (list)) =⇒#f
A pair (sometimes called a dotted pair) is a record structure with two fields called the car and cdr fields (for historical reasons). Pairs are created by the procedure cons. The car and cdr fields are accessed by the procedures car and cdr. The car and cdr fields are assigned by the procedures set-car! and set-cdr!.
Pairs are used primarily to represent lists. A list can be defined recursively as either the empty list or a pair whose cdr is a list. More precisely, the set of lists is defined as the smallest set X such that
• The empty list is in X.
• If list is in X , then any pair whose cdr field contains
list is also in X.
The objects in the car fields of successive pairs of a list are the elements of the list. For example, a two-element list is a pair whose car is the first element and whose cdr is a pair whose car is the second element and whose cdr is the empty list. The length of a list is the number of elements, which is the same as the number of pairs.
The empty list is a special object of its own type (it is not a pair); it has no elements and its length is zero.
Note: The above definitions imply that all lists have finite length and are terminated by the empty list.
The most general notation (external representation) for Scheme pairs is the “dotted” notation (c1 . c2) where c1 is the value of the car field and c2 is the value of the cdr field. For example (4 . 5) is a pair whose car is 4 and whose cdr is 5. Note that (4 . 5) is the external repre- sentation of a pair, not an expression that evaluates to a pair.
A more streamlined notation can be used for lists: the elements of the list are simply enclosed in parentheses and separated by spaces. The empty list is written () . For example,
(a b c d e)
and
#t #f ’#f
(not ’nil)
=⇒#f
(a . (b . (c . (d . (e . ())))))
library procedure
#f otherwise.
(boolean? #f)
(boolean? 0)
(boolean? ’())
6.3.2. Pairs and lists
=⇒ #t =⇒ #f =⇒ #f
6. Standard procedures 25 library procedure
26 Revised5 Scheme
are equivalent notations for a list of symbols.
A chain of pairs not ending in the empty list is called an improper list. Note that an improper list is not a list. The list and dotted notations can be combined to represent improper lists:
(a b c . d)
is equivalent to
(a . (b . (c . d)))
Whether a given pair is a list depends upon what is stored in the cdr field. When the set-cdr! procedure is used, an object can be a list one moment and not the next:
(car pair ) procedure
Returns the contents of the car field of pair. Note that it is an error to take the car of the empty list.
(define x (list ’a ’b ’c))
(cdr’((a)bcd)) (cdr ’(1 . 2)) (cdr ’())
=⇒ (bcd) =⇒ 2
=⇒ error
(define y
y
(list? y)
(set-cdr!
x
x)
x4)
=⇒ (abc) =⇒ #t
=⇒ unspecified =⇒ (a.4) =⇒ #t
=⇒ (a.4) =⇒ #f
=⇒ unspecified =⇒ #f
(eqv? x y)
y
(list? y)
(set-cdr! x x)
(list? x)
Within literal expressions and representations of ob- jects read by the read procedure, the forms ’⟨datum⟩, `⟨datum⟩, ,⟨datum⟩, and ,@⟨datum⟩ denote two-ele- ment lists whose first elements are the symbols quote, quasiquote, unquote, and unquote-splicing, respec- tively. The second element in each case is ⟨datum⟩. This convention is supported so that arbitrary Scheme pro- grams may be represented as lists. That is, according to Scheme’s grammar, every ⟨expression⟩ is also a ⟨datum⟩ (see section 7.1.2). Among other things, this permits the use of the read procedure to parse Scheme programs. See section 3.3.
(set-car! (f) 3)
(set-car! (g) 3)
(pair? obj)
Pair? returns #t if obj is a pair, and otherwise returns #f.
set-cdr! is unspecified. (caar pair)
(cadr pair) .
(cdddar pair) (cddddr pair)
library procedure library procedure
.
library procedure library procedure
(pair? ’(a . b))
(pair? ’(a b c))
(pair? ’())
(pair? ’#(a b))
(cons obj1 obj2)
=⇒ #t =⇒ #t =⇒ #f =⇒ #f
These procedures are compositions of car and cdr, where for example caddr could be defined by
(define caddr (lambda (x) (car (cdr (cdr x))))). Arbitrary compositions, up to four deep, are provided.
There are twenty-eight of these procedures in all.
(null? obj) library procedure
Returns #t if obj is the empty list, otherwise returns #f. (list? obj) library procedure
Returns #t if obj is a list, otherwise returns #f. By defini- tion, all lists have finite length and are terminated by the empty list.
Returns a newly allocated pair whose car is obj1 and whose cdr is obj2. The pair is guaranteed to be different (in the sense of eqv?) from every existing object.
(cons ’a ’())
(cons ’(a) ’(b c d))
(cons “a” ’(b c))
(cons ’a 3)
(cons ’(a b) ’c)
=⇒ (a)
=⇒ ((a)bcd) =⇒ (“a” b c) =⇒ (a.3)
=⇒ ((ab).c)
procedure
procedure
(car ’(a b c))
(car ’((a) b c d))
(car ’(1 . 2))
(car ’())
(cdr pair)
=⇒ a =⇒ (a) =⇒ 1 =⇒ error
Returns the contents of the cdr field of pair. Note that it is an error to take the cdr of the empty list.
(set-car! pair obj )
Stores obj in the car field of pair. The value returned by
set-car! is unspecified.
(define (f) (list ’not-a-constant-list))
(define (g) ’(constant-list))
=⇒ unspecified =⇒ error
(set-cdr! pair obj)
Stores obj in the cdr field of pair. The value returned by
procedure
procedure
procedure
(list? ’(a b c))
(list? ’()) =⇒#t (list? ’(a . b)) =⇒#f (let ((x (list ’a)))
Returns the kth element of list. (This is the same as the car of (list-tail list k).) It is an error if list has fewer than k elements.
(list-ref ’(a b c d) 2) =⇒ c (list-ref ’(a b c d)
(inexact->exact (round 1.8))) =⇒ c
(set-cdr! x x)
(list? x))
(list obj …)
Returns a newly allocated list of its arguments.
(list ’a (+ 3 4) ’c)
(list)
(length list)
Returns the length of list.
(length ’(a b c))
(length ’(a (b) (c d e)))
(length ’())
(append list …)
=⇒ (a7c) =⇒ ()
library procedure
=⇒ 3 =⇒ 3 =⇒ 0
libraryprocedure
(memq obj list) (memv obj list) (member obj list)
library procedure library procedure library procedure
Returns a list consisting of the elements of the first list followed by the elements of the other lists.
(memq ’a ’(a b c))
(memq ’b ’(a b c))
(memq ’a ’(b c d))
(memq (list ’a) ’(b (a) c))
(member (list ’a)
’(b (a) c))
(memq 101 ’(100 101 102))
(memv 101 ’(100 101 102))
(assq obj alist ) (assv obj alist ) (assoc obj alist )
=⇒ (abc) =⇒ (bc) =⇒ #f
=⇒ #f
=⇒ ((a) c) =⇒ unspecified =⇒ (101 102)
library procedure library procedure library procedure
(append ’(x) ’(y))
(append ’(a) ’(b c d))
(append ’(a (b)) ’((c)))
=⇒ (x =⇒ (a =⇒ (a
y) bcd) (b) (c))
The resulting list is always newly allocated, except that it shares structure with the last list argument. The last argument may actually be any object; an improper list results if the last argument is not a proper list.
=⇒ (a b c . d) =⇒ a
(reverse list )
Returns a newly allocated list consisting of the elements of
list in reverse order.
(reverse ’(a b c)) =⇒ (c b a)
(reverse ’(a (b c) d (e (f))))
=⇒ ((e(f))d(bc)a)
Alist (for “association list”) must be a list of pairs. These procedures find the first pair in alist whose car field is obj , and returns that pair. If no pair in alist has obj as its car, then #f (not the empty list) is returned. Assq uses eq? to compare obj with the car fields of the pairs in alist, while assv uses eqv? and assoc uses equal?.
(define e ’((a 1) (b 2) (c 3)))
=⇒ #f (assoc (list ’a) ’(((a)) ((b)) ((c))))
=⇒ ((a))
(assq 5 ’((2 3) (5 7) (11 13)))
=⇒ unspecified (assv 5 ’((2 3) (5 7) (11 13)))
=⇒ (57)
Rationale: Although they are ordinarily used as predicates, memq, memv, member, assq, assv, and assoc do not have question marks in their names because they return useful values rather than just #t or #f.
(append ’(a b) ’(c . d))
(append ’() ’a)
(list-tail list k)
library procedure
Returns the sublist of list obtained by omitting the first k elements. It is an error if list has fewer than k elements. List-tail could be defined by
(define list-tail
(lambda (x k)
(if (zero? k)
x
(list-tail (cdr x) (- k 1)))))
=⇒#t
(list-ref list k)
6. Standard procedures 27 library procedure
=⇒ #f
libraryprocedure
library procedure
These procedures return the first sublist of list whose car is obj, where the sublists of list are the non-empty lists returned by (list-tail list k) for k less than the length of list. If obj does not occur in list, then #f (not the empty list) is returned. Memq uses eq? to compare obj with the elements of list, while memv uses eqv? and member uses equal?.
(assq ’a e)
(assq ’b e)
(assq ’d e)
(assq (list ’a) ’(((a)) ((b)) ((c))))
=⇒ (a 1)
=⇒ (b 2)
=⇒ #f
28 Revised5 Scheme 6.3.3. Symbols
Symbols are objects whose usefulness rests on the fact that two symbols are identical (in the sense of eqv?) if and only if their names are spelled the same way. This is exactly the property needed to represent identifiers in programs, and so most implementations of Scheme use them internally for that purpose. Symbols are useful for many other applica- tions; for instance, they may be used the way enumerated values are used in Pascal.
The rules for writing a symbol are exactly the same as the rules for writing an identifier; see sections 2.1 and 7.1.1.
It is guaranteed that any symbol that has been returned as part of a literal expression, or read using the read proce- dure, and subsequently written out using the write proce- dure, will read back in as the identical symbol (in the sense of eqv?). The string->symbol procedure, however, can create symbols for which this write/read invariance may not hold because their names contain special characters or letters in the non-standard case.
Note: Some implementations of Scheme have a feature known as “slashification” in order to guarantee write/read invariance for all symbols, but historically the most important use of this feature has been to compensate for the lack of a string data type.
Some implementations also have “uninterned symbols”, which defeat write/read invariance even in implementations with slashification, and also generate exceptions to the rule that two symbols are the same if and only if their names are spelled the same.
The following examples assume that the implementation’s standard case is lower case:
(symbol->string ’flying-fish)
=⇒ “flying-fish”
(symbol? obj)
Returns #t if obj is a symbol, otherwise returns #f.
Returns the symbol whose name is string. This procedure can create symbols with names containing special charac- ters or letters in the non-standard case, but it is usually a bad idea to create such symbols because in some imple- mentations of Scheme they cannot be read as themselves. See symbol->string.
The following examples assume that the implementation’s standard case is lower case:
(eq? ’mISSISSIppi ’mississippi) =⇒ #t
(string->symbol “mISSISSIppi”)
=⇒ the symbol with name “mISSISSIppi”
(eq? ’bitBlt (string->symbol “bitBlt”)) =⇒ #f
(eq? ’JollyWog
(string->symbol
(symbol->string ’JollyWog))) =⇒ #t
(string=? “K. Harper, M.D.”
(symbol->string
(string->symbol “K. Harper, M.D.”))) =⇒ #t
6.3.4. Characters
Characters are objects that represent printed characters such as letters and digits. Characters are written using the notation #\⟨character⟩ or #\⟨character name⟩. For exam- ple:
#\a ; lower case letter
#\A ; upper case letter
#\( ; left parenthesis
#\ ; the space character
#\space ; the preferred way to write a space #\newline ; the newline character
Case is significant in #\⟨character⟩, but not in #\⟨character name⟩. If ⟨character⟩ in #\⟨character⟩ is alphabetic, then the character following ⟨character⟩ must be a delimiter character such as a space or parenthesis. This rule resolves the ambiguous case where, for example, the sequence of
(symbol? ’foo) =⇒#t (symbol? (car ’(a b))) =⇒#t (symbol? “bar”) =⇒#f (symbol? ’nil) =⇒#t (symbol? ’()) =⇒#f (symbol? #f) =⇒#f
(symbol->string symbol)
procedure
Returns the name of symbol as a string. If the symbol was part of an object returned as the value of a literal expression (section 4.1.2) or by a call to the read proce- dure, and its name contains alphabetic characters, then the string returned will contain characters in the implementa- tion’s preferred standard case—some implementations will prefer upper case, others lower case. If the symbol was re- turned by string->symbol, the case of characters in the string returned will be the same as the case in the string that was passed to string->symbol. It is an error to apply mutation procedures like string-set! to strings returned by this procedure.
procedure
=⇒ “martin” (string->symbol “Malvina”))
(symbol->string ’Martin)
(symbol->string
(string->symbol string)
=⇒ “Malvina” procedure
characters “#\space” could be taken to be either a repre- sentation of the space character or a representation of the character “#\s” followed by a representation of the symbol “pace.”
Characters written in the #\ notation are self-evaluating. That is, they do not have to be quoted in programs.
Some of the procedures that operate on characters ignore the difference between upper case and lower case. The pro- cedures that ignore case have “-ci” (for “case insensitive”) embedded in their names.
(char? obj ) procedure Returns #t if obj is a character, otherwise returns #f.
(char-alphabetic? char ) (char-numeric? char )
library procedure library procedure library procedure library procedure library procedure
(char=?
(char?
(char<=?
(char>=?
char1 char1 char1
char1 char1
char2 ) char2 ) char2 )
char2 ) char2 )
procedure procedure procedure procedure procedure
These procedures return #t if their arguments are alpha- betic, numeric, whitespace, upper case, or lower case char- acters, respectively, otherwise they return #f. The follow- ing remarks, which are specific to the ASCII character set, are intended only as a guide: The alphabetic characters are the 52 upper and lower case letters. The numeric charac- ters are the ten decimal digits. The whitespace characters are space, tab, line feed, form feed, and carriage return.
(char->integer char) procedure (integer->char n) procedure
Given a character, char->integer returns an exact inte- ger representation of the character. Given an exact inte- ger that is the image of a character under char->integer, integer->char returns that character. These procedures implement order-preserving isomorphisms between the set of characters under the char<=? ordering and some subset of the integers under the <= ordering. That is, if
(char<=?ab)=⇒#t and (<=xy)=⇒#t
and x and y are in the domain of integer->char, then
These procedures impose a total ordering on the set of characters. It is guaranteed that under this ordering:
• The upper case characters are in order. For example, (charinteger a) (char->integer b))
(char-ci=?
(char-ci?
(char-ci<=?
(char-ci>=?
char1 char1 char1
char1 char1
char2 ) char2 ) char2 )
char2 ) char2 )
library procedure library procedure library procedure library procedure library procedure
(char-upcase char) library procedure (char-downcase char ) library procedure
These procedures return a character char2 such that (char-ci=? char char2). In addition, if char is alpha- betic, then the result of char-upcase is upper case and the result of char-downcase is lower case.
6.3.5. Strings
Strings are sequences of characters. Strings are written as sequences of characters enclosed within doublequotes (“). A doublequote can be written inside a string only by escaping it with a backslash (\), as in
“The word \”recursion\” has many meanings.”
A backslash can be written inside a string only by escaping it with another backslash. Scheme does not specify the effect of a backslash within a string that is not followed by a doublequote or backslash.
These procedures are similar to char=? et cetera, but they treat upper case and lower case letters as the same. For example, (char-ci=? #\A #\a) returns #t. Some imple- mentations may generalize these procedures to take more than two arguments, as with the corresponding numerical predicates.
(char-whitespace?
(char-upper-case?
(char-lower-case?
char ) letter ) letter )
=⇒ #t (integer->char y)) =⇒ #t
(char<=? (integer->char x)
6. Standard procedures 29
30 Revised5 Scheme
A string constant may continue from one line to the next,
but the exact contents of such a string are unspecified.
The length of a string is the number of characters that it contains. This number is an exact, non-negative integer that is fixed when the string is created. The valid indexes of a string are the exact non-negative integers less than the length of the string. The first character of a string has index 0, the second has index 1, and so on.
In phrases such as “the characters of string beginning with index start and ending with index end,” it is understood that the index start is inclusive and the index end is ex- clusive. Thus if start and end are the same index, a null substring is referred to, and if start is zero and end is the length of string, then the entire string is referred to.
Some of the procedures that operate on strings ignore the difference between upper and lower case. The versions that ignore case have “-ci” (for “case insensitive”) embedded in their names.
(string? obj) procedure Returns #t if obj is a string, otherwise returns #f.
(string=? string1 string2) library procedure (string-ci=? string1 string2) library procedure
Returns #t if the two strings are the same length and con- tain the same characters in the same positions, otherwise returns #f. String-ci=? treats upper and lower case let- ters as though they were the same character, but string=? treats upper and lower case as distinct characters.
(make-string k) (make-string k char)
procedure procedure
These procedures are the lexicographic extensions to strings of the corresponding orderings on characters. For example, stringlist string) (list->string list)
library procedure library procedure
(string-set! (f) 0 #\?)
(string-set! (g) 0 #\?)
(string-set! (symbol->string ’immutable)
=⇒ unspecified =⇒ error
String->list returns a newly allocated list of the charac- ters that make up the given string. List->string returns a newly allocated string formed from the characters in the list list, which must be a list of characters. String->list and list->string are inverses so far as equal? is con- cerned.
(string-copy string) library procedure Returns a newly allocated copy of the given string.
(string?
(string<=?
(string>=?
(string-ci?
(string-ci<=?
(string-ci>=?
string1 string1
string2 ) string2 )
string2 ) string2 )
library procedure library procedure library procedure library procedure library procedure library procedure library procedure library procedure
string1 string1
string1 string1
string1 string1
string2 ) string2 )
string2 ) string2 )
(string-fill! string char ) library procedure Stores char in every element of the given string and returns
an unspecified value.
6.3.6. Vectors
Vectors are heterogenous structures whose elements are in- dexed by integers. A vector typically occupies less space than a list of the same length, and the average time re- quired to access a randomly chosen element is typically less for the vector than for the list.
The length of a vector is the number of elements that it contains. This number is a non-negative integer that is fixed when the vector is created. The valid indexes of a vector are the exact non-negative integers less than the length of the vector. The first element in a vector is indexed by zero, and the last element is indexed by one less than the length of the vector.
Vectors are written using the notation #(obj . . . ). For example, a vector of length 3 containing the number zero in element 0, the list (2 2 2 2) in element 1, and the string “Anna” in element 2 can be written as following:
#(0 (2 2 2 2) “Anna”)
Note that this is the external representation of a vector, not an expression evaluating to a vector. Like list constants, vector constants must be quoted:
’#(0 (2 2 2 2) “Anna”)
=⇒ #(0(2222)”Anna”)
(vector-ref ’#(1 1 2 3 5 8 13 21)
5)
=⇒ 8
(vector-ref ’#(1 1 2 3 5 8 13 21)
(let ((i (round (* 2 (acos -1)))))
(if (inexact? i)
(vector? obj)
Returns #t if obj is a vector, otherwise returns #f.
(make-vector k) procedure (make-vector k fill) procedure
Returns a newly allocated vector of k elements. If a second argument is given, then each element is initialized to fill. Otherwise the initial contents of each element is unspeci- fied.
(vector obj . . . ) library procedure
Returns a newly allocated vector whose elements contain the given arguments. Analogous to list.
k must be a valid index of vector. Vector-set! stores obj in element k of vector . The value returned by vector-set! is unspecified.
(let ((vec (vector 0 ’(2 2 2 2) “Anna”)))
(vector-set! vec 1 ’(“Sue” “Sue”))
vec)
=⇒ #(0 (“Sue” “Sue”) “Anna”) (vector-set! ’#(0 1 2) 1 “doe”)
=⇒ error ; constant vector (vector->list vector) library procedure
(list->vector list) library procedure
Vector->list returns a newly allocated list of the objects contained in the elements of vector. List->vector returns a newly created vector initialized to the elements of the list list .
(vector->list ’#(dah dah didah)) =⇒ (dah dah didah)
(list->vector ’(dididit dah)) =⇒ #(dididit dah)
(vector-fill! vector fill)
Stores fill in every element of vector. The value returned
by vector-fill! is unspecified. 6.4. Control features
This chapter describes various primitive procedures which control the flow of program execution in special ways. The procedure? predicate is also described here.
(procedure? obj) procedure Returns #t if obj is a procedure, otherwise returns #f.
(procedure? car) =⇒ #t (procedure? ’car) =⇒ #f (procedure? (lambda (x) (* x x)))
=⇒ #t (procedure? ’(lambda (x) (* x x)))
=⇒ #f (call-with-current-continuation procedure?)
=⇒ #t
(vector ’a ’b ’c)
=⇒ #(a b c) procedure
(vector-length vector)
Returns the number of elements in vector as an exact in-
teger.
(vector-ref vector k)
procedure
k must be a valid index of vector. Vector-ref returns the contents of element k of vector.
procedure
(inexact->exact i)
i))) =⇒ 13
(vector-set! vector k obj)
procedure
6. Standard procedures 31
library procedure
32 Revised5 Scheme
(apply proc arg1 . . . args) procedure
Proc must be a procedure and args must be a list. Calls proc with the elements of the list (append (list arg1 …) args) as the actual arguments.
computed and returned. The value of the promise is cached (or “memoized”) so that if it is forced a second time, the previously computed value is returned.
(force (delay (+ 1 2))) =⇒ 3 (let ((p (delay (+ 1 2))))
(list (force p) (force p)))
=⇒ (33)
(define a-stream
(letrec ((next
(lambda (n)
(cons n (delay (next (+ n 1)))))))
(next 0)))
(define head car)
(define tail
(lambda (stream) (force (cdr stream))))
(head (tail (tail a-stream)))
=⇒ 2
Force and delay are mainly intended for programs written in functional style. The following examples should not be considered to illustrate good programming style, but they illustrate the property that only one value is computed for a promise, no matter how many times it is forced.
(define count 0)
(define p
(delay (begin (set! count (+ count 1))
(if (> count x)
(apply + (list 3 4))
(define compose
(lambda (f g)
(lambda args
(f (apply g args)))))
((compose sqrt *) 12 75)
(map proc list1 list2 . . . )
=⇒ 7
=⇒ 30
library procedure
The lists must be lists, and proc must be a procedure taking as many arguments as there are lists and returning a single value. If more than one list is given, then they must all be the same length. Map applies proc element-wise to the elements of the lists and returns a list of the results, in order. The dynamic order in which proc is applied to the elements of the lists is unspecified.
(map cadr ’((a b) (d e) (g h))) =⇒ (beh)
(map (lambda (n) (expt n n)) ’(12345))
=⇒ (14272563125)
(map+’(123)’(456)) =⇒ (579)
(let ((count 0))
(map (lambda (ignored)
count
(force p)))))
(for-each
’(ab)))
proc list1
list2
. . . )
=⇒ (12)or(21)
library procedure
Here is a possible implementation of delay and force. Promises are implemented here as procedures of no argu- ments, and force simply calls its argument:
(define force
(lambda (object)
(object)))
We define the expression
(delay ⟨expression⟩)
to have the same meaning as the procedure call
(make-promise (lambda () ⟨expression⟩)) as follows
(define-syntax delay
(syntax-rules ()
((delay expression)
(make-promise (lambda () expression))))),
(set! count (+ count 1))
count)
=⇒ a promise, still (force p)) =⇒ 6
The arguments to for-each are like the arguments to map, but for-each calls proc for its side effects rather than for its values. Unlike map, for-each is guaranteed to call proc on the elements of the lists in order from the first ele- ment(s) to the last, and the value returned by for-each is unspecified.
(let ((v (make-vector 5)))
(for-each (lambda (i)
v)
(vector-set! v i (* i i)))
’(0 1 2 3 4))
=⇒ #(014916) library procedure
(force promise)
Forces the value of promise (see delay, section 4.2.5). If
no value has been computed for the promise, then a value is
(define x 5) p
(force
p
(begin
=⇒ a promise p) =⇒6
(set! x 10)
where make-promise is defined as follows:
(define make-promise
(lambda (proc)
(let ((result-ready? #f)
(result #f))
(lambda ()
(if result-ready?
result
(let ((x (proc)))
(if result-ready?
result
(begin (set! result-ready? #t)
(set! result x)
result))))))))
A promise may refer to its own value, as in the last example above. Forcing such a promise may cause the promise to be forced a second time before the value of the first force has been computed. This complicates the definition of make-promise.
Various extensions to this semantics of delay and force are supported in some implementations:
• Calling force on an object that is not a promise may simply return the object.
• It may be the case that there is no means by which a promise can be operationally distinguished from its forced value. That is, expressions like the following may evaluate to either #t or to #f, depending on the implementation:
• Some implementations may implement “implicit forc- ing,” where the value of a promise is forced by primi- tive procedures like cdr and +:
call-with-current-continuation. Except for continua- tions created by the call-with-values procedure, all con- tinuations take exactly one value. The effect of passing no value or more than one value to continuations that were not created by call-with-values is unspecified.
The escape procedure that is passed to proc has unlimited extent just like any other procedure in Scheme. It may be stored in variables or data structures and may be called as many times as desired.
The following examples show only the most common ways in which call-with-current-continuation is used. If all real uses were as simple as these examples, there would be no need for a procedure with the power of call-with-current-continuation.
(call-with-current-continuation
(lambda (exit)
(for-each (lambda (x)
(if (negative? x)
(exit x)))
’(54 0 37 -3 245 19)) #t)) =⇒ -3
(define list-length
(lambda (obj)
(call-with-current-continuation
(lambda (return)
Rationale:
(eqv? (delay 1) 1)
(pair? (delay (cons 1 2))) =⇒ unspecified
(+ (delay (* 3 7)) 13)
=⇒ 34
A common use of call-with-current-continuation is for structured, non-local exits from loops or procedure bodies, but in fact call-with-current-continuation is extremely useful for implementing a wide variety of advanced control structures.
Whenever a Scheme expression is evaluated there is a contin- uation wanting the result of the expression. The continuation represents an entire (default) future for the computation. If the expression is evaluated at top level, for example, then the con- tinuation might take the result, print it on the screen, prompt for the next input, evaluate it, and so on forever. Most of the time the continuation includes actions specified by user code, as in a continuation that will take the result, multiply it by the value stored in a local variable, add seven, and give the answer to the top level continuation to be printed. Normally these ubiquitous continuations are hidden behind the scenes and pro- grammers do not think much about them. On rare occasions, however, a programmer may need to deal with continuations ex- plicitly. Call-with-current-continuation allows Scheme pro-
(call-with-current-continuation proc)
procedure
Proc must be a procedure of one argument. The procedure call-with-current-continuation packages up the cur- rent continuation (see the rationale below) as an “escape procedure” and passes it as an argument to proc. The es- cape procedure is a Scheme procedure that, if it is later called, will abandon whatever continuation is in effect at that later time and will instead use the continuation that was in effect when the escape procedure was created. Call- ing the escape procedure may cause the invocation of before and after thunks installed using dynamic-wind.
The escape procedure accepts the same number of ar- guments as the continuation to the original call to
=⇒ unspecified
(letrec ((r
(lambda
(cond
(r obj))))))
(list-length ’(1 2 3 4))
(list-length ’(a b . c))
Rationale:
(obj)
((null? obj) 0)
((pair? obj)
(+ (r (cdr obj)) 1))
(else (return #f))))))
=⇒ 4 =⇒ #f
6. Standard procedures 33
34 Revised5 Scheme
grammers to do that by creating a procedure that acts just like
the current continuation.
Most programming languages incorporate one or more special- purpose escape constructs with names like exit, return, or even goto. In 1965, however, Peter Landin [16] invented a general purpose escape operator called the J-operator. John Reynolds [24] described a simpler but equally powerful con- struct in 1972. The catch special form described by Sussman and Steele in the 1975 report on Scheme is exactly the same as Reynolds’s construct, though its name came from a less general construct in MacLisp. Several Scheme implementors noticed that the full power of the catch construct could be provided by a procedure instead of by a special syntactic construct, and the name call-with-current-continuation was coined in 1982. This name is descriptive, but opinions differ on the merits of such a long name, and some people use the name call/cc in- stead.
(values obj …) procedure
Delivers all of its arguments to its continuation. Except for continuations created by the call-with-values pro- cedure, all continuations take exactly one value. Values might be defined as follows:
(define (values . things)
(call-with-current-continuation
(lambda (cont) (apply cont things))))
(call-with-values producer consumer) procedure
Calls its producer argument with no values and a contin- uation that, when passed some values, calls the consumer procedure with those values as arguments. The continua- tion for the call to consumer is the continuation of the call to call-with-values.
(call-with-values (lambda () (values 4 5))
(lambda (a b) b))
Scheme, because of call-with-current-continuation, the dynamic extent of a call may not be a single, connected time period. It is defined as follows:
• The dynamic extent is entered when execution of the body of the called procedure begins.
• The dynamic extent is also entered when exe- cution is not within the dynamic extent and a continuation is invoked that was captured (using call-with-current-continuation) during the dy- namic extent.
• It is exited when the called procedure returns.
• It is also exited when execution is within the dynamic extent and a continuation is invoked that was captured while not within the dynamic extent.
If a second call to dynamic-wind occurs within the dynamic extent of the call to thunk and then a continuation is in- voked in such a way that the afters from these two invoca- tions of dynamic-wind are both to be called, then the after associated with the second (inner) call to dynamic-wind is called first.
If a second call to dynamic-wind occurs within the dy- namic extent of the call to thunk and then a continua- tion is invoked in such a way that the befores from these two invocations of dynamic-wind are both to be called, then the before associated with the first (outer) call to dynamic-wind is called first.
If invoking a continuation requires calling the before from one call to dynamic-wind and the after from another, then the after is called first.
The effect of using a captured continuation to enter or exit the dynamic extent of a call to before or after is undefined.
(let ((path ’())
(c #f))
(let ((add (lambda (s)
(set! path (cons s path)))))
(dynamic-wind
(lambda () (add ’connect))
(lambda ()
(add (call-with-current-continuation
(lambda (c0)
(set! c c0)
’talk1))))
(lambda () (add ’disconnect)))
(if (< (length path) 4)
(c ’talk2)
(reverse path))))
=⇒ (connect talk1 disconnect connect talk2 disconnect)
(call-with-values * -)
=⇒ 5 =⇒ -1
(dynamic-wind before thunk after)
procedure
Calls thunk without arguments, returning the result(s) of this call. Before and after are called, also without ar- guments, as required by the following rules (note that in the absence of calls to continuations captured using call-with-current-continuation the three arguments are called once each, in order). Before is called whenever execution enters the dynamic extent of the call to thunk and after is called whenever it exits that dynamic extent. The dynamic extent of a procedure call is the period be- tween when the call is initiated and when it returns. In
6.5. Eval
(eval expression environment-specifier) procedure
Evaluates expression in the specified environment and re- turns its value. Expression must be a valid Scheme expres- sion represented as data, and environment-specifier must be a value returned by one of the three procedures de- scribed below. Implementations may extend eval to allow non-expression programs (definitions) as the first argument and to allow other values as environments, with the re- striction that eval is not allowed to create new bindings in the environments associated with null-environment or scheme-report-environment.
(eval ’(* 7 3) (scheme-report-environment 5)) =⇒ 21
6.6. Input and output
6.6.1. Ports
Ports represent input and output devices. To Scheme, an input port is a Scheme object that can deliver characters upon command, while an output port is a Scheme object that can accept characters.
(call-with-input-file string proc) library procedure (call-with-output-file string proc) library procedure
String should be a string naming a file, and proc should be a procedure that accepts one argument. For call-with-input-file, the file should already exist; for call-with-output-file, the effect is unspecified if the file already exists. These procedures call proc with one ar- gument: the port obtained by opening the named file for input or output. If the file cannot be opened, an error is signalled. If proc returns, then the port is closed automati- cally and the value(s) yielded by the proc is(are) returned. If proc does not return, then the port will not be closed automatically unless it is possible to prove that the port will never again be used for a read or write operation.
Rationale: Because Scheme’s escape procedures have un- limited extent, it is possible to escape from the current con- tinuation but later to escape back in. If implementations were permitted to close the port on any escape from the current continuation, then it would be impossible to write portable code using both call-with-current-continuation and call-with-input-file or call-with-output-file.
(let ((f (eval ’(lambda (f x) (f x x))
(null-environment 5))))
(f + 10))
=⇒ 20
(scheme-report-environment version) (null-environment version)
procedure procedure
Version must be the exact integer 5, corresponding to this revision of the Scheme report (the Revised5 Report on Scheme). Scheme-report-environment returns a specifier for an environment that is empty except for all bindings de- fined in this report that are either required or both optional and supported by the implementation. Null-environment returns a specifier for an environment that is empty except for the (syntactic) bindings for all syntactic keywords de- fined in this report that are either required or both optional and supported by the implementation.
Other values of version can be used to specify environments matching past revisions of this report, but their support is not required. An implementation will signal an error if version is neither 5 nor another value supported by the implementation.
The effect of assigning (through the use of eval) a vari- able bound in a scheme-report-environment (for exam- ple car) is unspecified. Thus the environments specified by scheme-report-environment may be immutable.
(interaction-environment) optional procedure
This procedure returns a specifier for the environment that contains implementation-defined bindings, typically a su- perset of those listed in the report. The intent is that this procedure will return the environment in which the imple- mentation would evaluate expressions dynamically typed by the user.
(input-port? obj) (output-port? obj)
procedure procedure
Returns #t if obj is an input port or output port respec- tively, otherwise returns #f.
(current-input-port) procedure (current-output-port) procedure
Returns the current default input or output port.
(with-input-from-file string thunk)
optional procedure
(with-output-to-file string thunk)
optional procedure
String should be a string naming a file, and proc should be a procedure of no arguments. For with-input-from-file, the file should already exist; for with-output-to-file, the effect is unspecified if the file already exists. The file is opened for input or output, an input or output port connected to it is made the default value returned by current-input-port or current-output-port (and is
6. Standard procedures 35
36 Revised5 Scheme
used by (read), (write obj ), and so forth), and the thunk is called with no arguments. When the thunk returns, the port is closed and the previous default is restored. With-input-from-file and with-output-to-file re- turn(s) the value(s) yielded by thunk. If an escape pro- cedure is used to escape from the continuation of these procedures, their behavior is implementation dependent.
(open-input-file filename) procedure
Takes a string naming an existing file and returns an input port capable of delivering characters from the file. If the file cannot be opened, an error is signalled.
(read-char) procedure (read-char port ) procedure
Returns the next character available from the input port, updating the port to point to the following character. If no more characters are available, an end of file object is returned. Port may be omitted, in which case it defaults to the value returned by current-input-port.
(open-output-file filename)
procedure
Returns the next character available from the input port, without updating the port to point to the following char- acter. If no more characters are available, an end of file object is returned. Port may be omitted, in which case it defaults to the value returned by current-input-port.
Note: The value returned by a call to peek-char is the same as the value that would have been returned by a call to read-char with the same port. The only difference is that the very next call to read-char or peek-char on that port will return the value returned by the preceding call to peek-char. In particular, a call to peek-char on an interactive port will hang waiting for input whenever a call to read-char would have hung.
(eof-object? obj ) procedure
Returns #t if obj is an end of file object, otherwise returns #f. The precise set of end of file objects will vary among implementations, but in any case no end of file object will ever be an object that can be read in using read.
(char-ready?) procedure (char-ready? port) procedure
Returns #t if a character is ready on the input port and returns #f otherwise. If char-ready returns #t then the next read-char operation on the given port is guaranteed not to hang. If the port is at end of file then char-ready? returns #t. Port may be omitted, in which case it defaults to the value returned by current-input-port.
Rationale: Char-ready? exists to make it possible for a pro- gram to accept characters from interactive ports without getting stuck waiting for input. Any input editors associated with such ports must ensure that characters whose existence has been as- serted by char-ready? cannot be rubbed out. If char-ready? were to return #f at end of file, a port at end of file would be indistinguishable from an interactive port that has no ready characters.
6.6.3. Output
Takes a string naming an output file to be created and returns an output port capable of writing characters to a new file by that name. If the file cannot be opened, an error is signalled. If a file with the given name already exists, the effect is unspecified.
(close-input-port port) (close-output-port port)
procedure procedure
Closes the file associated with port, rendering the port in- capable of delivering or accepting characters. These rou- tines have no effect if the file has already been closed. The value returned is unspecified.
6.6.2. Input
(read) (read port)
library procedure library procedure
Read converts external representations of Scheme objects into the objects themselves. That is, it is a parser for the nonterminal ⟨datum⟩ (see sections 7.1.2 and 6.3.2). Read returns the next object parsable from the given input port, updating port to point to the first character past the end of the external representation of the object.
If an end of file is encountered in the input before any char- acters are found that can begin an object, then an end of file object is returned. The port remains open, and fur- ther attempts to read will also return an end of file object. If an end of file is encountered after the beginning of an object’s external representation, but the external represen- tation is incomplete and therefore not parsable, an error is signalled.
The port argument may be omitted, in which case it de- faults to the value returned by current-input-port. It is an error to read from a closed port.
(peek-char) (peek-char port )
procedure procedure
(write obj ) library procedure (write obj port) library procedure
Writes a written representation of obj to the given port. Strings that appear in the written representation are en- closed in doublequotes, and within those strings backslash and doublequote characters are escaped by backslashes. Character objects are written using the #\ notation. Write returns an unspecified value. The port argument may be omitted, in which case it defaults to the value returned by current-output-port.
sequentially. It is unspecified whether the results of the expressions are printed. The load procedure does not affect the values returned by current-input-port and current-output-port. Load returns an unspecified value.
Rationale: For portability, load must operate on source files. Its operation on other kinds of files necessarily varies among implementations.
(display obj ) (display obj port )
library procedure library procedure
Filename must be a string naming an output file to be cre- ated. The effect of transcript-on is to open the named file for output, and to cause a transcript of subsequent interaction between the user and the Scheme system to be written to the file. The transcript is ended by a call to transcript-off, which closes the transcript file. Only one transcript may be in progress at any time, though some implementations may relax this restriction. The values re- turned by these procedures are unspecified.
Writes a representation of obj to the given port. Strings that appear in the written representation are not enclosed in doublequotes, and no characters are escaped within those strings. Character objects appear in the represen- tation as if written by write-char instead of by write. Display returns an unspecified value. The port argument may be omitted, in which case it defaults to the value re- turned by current-output-port.
Rationale: Write is intended for producing machine-readable output and display is for producing human-readable output. Implementations that allow “slashification” within symbols will probably want write but not display to slashify funny charac- ters in symbols.
(newline) library procedure (newline port) library procedure
Writes an end of line to port. Exactly how this is done differs from one operating system to another. Returns an unspecified value. The port argument may be omit- ted, in which case it defaults to the value returned by current-output-port.
(write-char char) procedure (write-char char port) procedure
Writes the character char (not an external representa- tion of the character) to the given port and returns an unspecified value. The port argument may be omit- ted, in which case it defaults to the value returned by current-output-port.
6.6.4. System interface
Questions of system interface generally fall outside of the domain of this report. However, the following operations are important enough to deserve description here.
(load filename) optional procedure
Filename should be a string naming an existing file con- taining Scheme source code. The load procedure reads ex- pressions and definitions from the file and evaluates them
(transcript-on filename) (transcript-off)
optional procedure optional procedure
6. Standard procedures 37
38 Revised5 Scheme
7. Formal syntax and semantics
This chapter provides formal descriptions of what has al- ready been described informally in previous chapters of this report.
7.1. Formal syntax
This section provides a formal syntax for Scheme written in an extended BNF.
All spaces in the grammar are for legibility. Case is insignif- icant; for example, #x1A and #X1a are equivalent. ⟨empty⟩ stands for the empty string.
The following extensions to BNF are used to make the de- scription more concise: ⟨thing⟩* means zero or more occur- rences of ⟨thing⟩; and ⟨thing⟩+ means at least one ⟨thing⟩.
7.1.1. Lexical structure
This section describes how individual tokens (identifiers, numbers, etc.) are formed from sequences of characters. The following sections describe how expressions and pro- grams are formed from sequences of tokens.
⟨Intertoken space⟩ may occur on either side of any token, but not within a token.
Tokens which require implicit termination (identifiers, numbers, characters, and dot) may be terminated by any ⟨delimiter⟩, but not necessarily by anything else.
The following five characters are reserved for future exten- sions to the language: [ ] { } |
⟨token⟩ −→ ⟨identifier⟩ | ⟨boolean⟩ | ⟨number⟩ | ⟨character⟩ | ⟨string⟩
| ( | ) | #( | ’ | ` | , | ,@ | .
⟨delimiter⟩ −→ ⟨whitespace⟩ | ( | ) | " | ; ⟨whitespace⟩ −→ ⟨space or newline⟩
⟨comment⟩ −→ ; ⟨all subsequent characters up to a
line break⟩
⟨atmosphere⟩ −→ ⟨whitespace⟩ | ⟨comment⟩
⟨intertoken space⟩ −→ ⟨atmosphere⟩*
⟨identifier⟩ −→ ⟨initial⟩ ⟨subsequent⟩* | ⟨peculiar identifier⟩
⟨initial⟩ −→ ⟨letter⟩ | ⟨special initial⟩ ⟨letter⟩ −→ a | b | c | ... | z
⟨special initial⟩ −→ ! | $ | % | & | * | / | : | < | = |>|?|^|_|~
⟨subsequent⟩ −→ ⟨initial⟩ | ⟨digit⟩ | ⟨special subsequent⟩
⟨digit⟩ −→ 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 ⟨special subsequent⟩ −→ + | – | . | @
⟨peculiar identifier⟩ −→ + | – | …
⟨syntactic keyword⟩ −→ ⟨expression keyword⟩ | else | => | define
| unquote | unquote-splicing
⟨expression keyword⟩ −→ quote | lambda | if | set! | begin | cond | and | or | case | let | let* | letrec | do | delay
| quasiquote
⟨variable⟩ −→ ⟨any ⟨identifier⟩ that isn’t also a ⟨syntactic keyword⟩⟩
⟨boolean⟩ −→ #t | #f
⟨character⟩ −→ #\ ⟨any character⟩
| #\ ⟨character name⟩
⟨character name⟩ −→ space | newline
⟨string⟩ −→ ” ⟨string element⟩* ”
⟨string element⟩ −→ ⟨any character other than ” or \⟩
| \” | \\
⟨number⟩ −→ ⟨num 2⟩| ⟨num 8⟩
| ⟨num 10⟩| ⟨num 16⟩
The following rules for ⟨num R⟩, ⟨complex R⟩, ⟨real R⟩, ⟨ureal R⟩, ⟨uinteger R⟩, and ⟨prefix R⟩ should be repli- cated for R = 2, 8, 10, and 16. There are no rules for ⟨decimal 2⟩, ⟨decimal 8⟩, and ⟨decimal 16⟩, which means that numbers containing decimal points or exponents must be in decimal radix.
⟨num R⟩ −→ ⟨prefix R⟩ ⟨complex R⟩
⟨complex R⟩ −→ ⟨real R⟩ | ⟨real R⟩ @ ⟨real R⟩
| ⟨realR⟩ + ⟨urealR⟩ i | ⟨realR⟩ – ⟨urealR⟩ i | ⟨real R⟩ + i | ⟨real R⟩ – i
| + ⟨ureal R⟩ i | – ⟨ureal R⟩ i | + i | – i
⟨real R⟩ −→ ⟨sign⟩ ⟨ureal R⟩ ⟨ureal R⟩ −→ ⟨uinteger R⟩
| ⟨uinteger R⟩ / ⟨uinteger R⟩
| ⟨decimal R⟩
⟨decimal 10⟩ −→ ⟨uinteger 10⟩ ⟨suffix⟩
| . ⟨digit 10⟩+ #* ⟨suffix⟩
| ⟨digit 10⟩+ . ⟨digit 10⟩* #* ⟨suffix⟩ | ⟨digit 10⟩+ #+ . #* ⟨suffix⟩
⟨uinteger R⟩ −→ ⟨digit R⟩+ #* ⟨prefix R⟩ −→ ⟨radix R⟩ ⟨exactness⟩
| ⟨exactness⟩ ⟨radix R⟩
⟨suffix⟩ −→ ⟨empty⟩
| ⟨exponent marker⟩ ⟨sign⟩ ⟨digit 10⟩+
⟨exponentmarker⟩−→e|s|f|d|l ⟨sign⟩ −→ ⟨empty⟩ | + | –
⟨exactness⟩ −→ ⟨empty⟩ | #i | #e
⟨radix 2⟩ −→ #b
⟨radix 8⟩ −→ #o
⟨radix 10⟩ −→ ⟨empty⟩ | #d
⟨radix 16⟩ −→ #x
⟨digit 2⟩ −→ 0 | 1
⟨digit 8⟩ −→ 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 ⟨digit 10⟩ −→ ⟨digit⟩
⟨digit 16⟩ −→ ⟨digit 10⟩ | a | b | c | d | e | f
7.1.2. External representations
⟨Datum⟩ is what the read procedure (section 6.6.2) suc- cessfully parses. Note that any string that parses as an ⟨expression⟩ will also parse as a ⟨datum⟩.
⟨datum⟩ −→ ⟨simple datum⟩ | ⟨compound datum⟩ ⟨simple datum⟩ −→ ⟨boolean⟩ | ⟨number⟩
| ⟨character⟩ | ⟨string⟩ | ⟨symbol⟩
⟨symbol⟩ −→ ⟨identifier⟩
⟨compound datum⟩ −→ ⟨list⟩ | ⟨vector⟩
⟨list⟩ −→ (⟨datum⟩*) | (⟨datum⟩+ . ⟨datum⟩)
| ⟨abbreviation⟩
⟨abbreviation⟩ −→ ⟨abbrev prefix⟩ ⟨datum⟩ ⟨abbrev prefix⟩ −→ ’ | ` | , | ,@
⟨vector⟩ −→ #(⟨datum⟩*)
7.1.3. Expressions
⟨expression⟩ −→ ⟨variable⟩ | ⟨literal⟩
| ⟨procedure call⟩
| ⟨lambda expression⟩ | ⟨conditional⟩
| ⟨assignment⟩
| ⟨derived expression⟩ | ⟨macro use⟩
| ⟨macro block⟩
⟨literal⟩ −→ ⟨quotation⟩ | ⟨self-evaluating⟩ ⟨self-evaluating⟩ −→ ⟨boolean⟩ | ⟨number⟩
| ⟨character⟩ | ⟨string⟩
⟨quotation⟩ −→ ’⟨datum⟩ | (quote ⟨datum⟩) ⟨procedure call⟩ −→ (⟨operator⟩ ⟨operand⟩*) ⟨operator⟩ −→ ⟨expression⟩
⟨operand⟩ −→ ⟨expression⟩
⟨lambda expression⟩ −→ (lambda ⟨formals⟩ ⟨body⟩) ⟨formals⟩ −→ (⟨variable⟩*) | ⟨variable⟩
| (⟨variable⟩+ . ⟨variable⟩)
⟨body⟩ −→ ⟨definition⟩* ⟨sequence⟩ ⟨sequence⟩ −→ ⟨command⟩* ⟨expression⟩ ⟨command⟩ −→ ⟨expression⟩
⟨conditional⟩ −→ (if ⟨test⟩ ⟨consequent⟩ ⟨alternate⟩) ⟨test⟩ −→ ⟨expression⟩
⟨consequent⟩ −→ ⟨expression⟩
⟨alternate⟩ −→ ⟨expression⟩ | ⟨empty⟩
⟨assignment⟩ −→ (set! ⟨variable⟩ ⟨expression⟩)
⟨derived expression⟩ −→ (cond ⟨cond clause⟩+ )
| (cond ⟨cond clause⟩* (else ⟨sequence⟩)) | (case ⟨expression⟩
⟨case clause⟩+)
| (case ⟨expression⟩
⟨case clause⟩*
(else ⟨sequence⟩)) | (and ⟨test⟩*)
| (or ⟨test⟩*)
| (let (⟨binding spec⟩*) ⟨body⟩)
| (let ⟨variable⟩ (⟨binding spec⟩*) ⟨body⟩) | (let* (⟨binding spec⟩*) ⟨body⟩)
| (letrec (⟨binding spec⟩*) ⟨body⟩)
| (begin ⟨sequence⟩)
| (do (⟨iteration spec⟩*)
(⟨test⟩ ⟨do result⟩) ⟨command⟩*)
| (delay ⟨expression⟩) | ⟨quasiquotation⟩
⟨cond clause⟩ −→ (⟨test⟩ ⟨sequence⟩) | (⟨test⟩)
| (⟨test⟩ => ⟨recipient⟩)
⟨recipient⟩ −→ ⟨expression⟩
⟨case clause⟩ −→ ((⟨datum⟩*) ⟨sequence⟩) ⟨binding spec⟩ −→ (⟨variable⟩ ⟨expression⟩) ⟨iteration spec⟩ −→ (⟨variable⟩ ⟨init⟩ ⟨step⟩)
| (⟨variable⟩ ⟨init⟩)
⟨init⟩ −→ ⟨expression⟩
⟨step⟩ −→ ⟨expression⟩
⟨do result⟩ −→ ⟨sequence⟩ | ⟨empty⟩
⟨macro use⟩ −→ (⟨keyword⟩ ⟨datum⟩*) ⟨keyword⟩ −→ ⟨identifier⟩
⟨macro block⟩ −→
(let-syntax (⟨syntax spec⟩*) ⟨body⟩)
| (letrec-syntax (⟨syntax spec⟩*) ⟨body⟩)
⟨syntax spec⟩ −→ (⟨keyword⟩ ⟨transformer spec⟩)
7.1.4. Quasiquotations
The following grammar for quasiquote expressions is not context-free. It is presented as a recipe for generating an infinite number of production rules. Imagine a copy of the following rules for D = 1,2,3,…. D keeps track of the nesting depth.
⟨quasiquotation⟩ −→ ⟨quasiquotation 1⟩ ⟨qq template 0⟩ −→ ⟨expression⟩
7. Formal syntax and semantics 39
40 Revised5 Scheme
⟨quasiquotation D⟩ −→ `⟨qq template D⟩ | (quasiquote ⟨qq template D⟩)
⟨qq template D⟩ −→ ⟨simple datum⟩ | ⟨list qq template D⟩
| ⟨vector qq template D⟩
| ⟨unquotation D⟩
⟨list qq template D⟩ −→ (⟨qq template or splice D⟩*) | (⟨qq template or splice D⟩+ . ⟨qq template D⟩) | ’⟨qq template D⟩
| ⟨quasiquotation D + 1⟩
⟨vector qq template D⟩ −→ #(⟨qq template or splice D⟩*) ⟨unquotation D⟩ −→ ,⟨qq template D − 1⟩
| (unquote ⟨qq template D − 1⟩)
⟨qq template or splice D⟩ −→ ⟨qq template D⟩
| ⟨splicing unquotation D⟩
⟨splicing unquotation D⟩ −→ ,@⟨qq template D − 1⟩
| (unquote-splicing ⟨qq template D − 1⟩)
In ⟨quasiquotation⟩s, a ⟨list qq template D⟩ can some- times be confused with either an ⟨unquotation D⟩ or a ⟨splicing unquotation D⟩. The interpretation as an ⟨unquotation⟩ or ⟨splicing unquotation D⟩ takes prece- dence.
7.1.5. Transformers
⟨transformer spec⟩ −→
(syntax-rules (⟨identifier⟩*) ⟨syntax rule⟩*)
⟨syntax rule⟩ −→ (⟨pattern⟩ ⟨template⟩) ⟨pattern⟩ −→ ⟨pattern identifier⟩
| (⟨pattern⟩*)
| (⟨pattern⟩+ . ⟨pattern⟩)
| (⟨pattern⟩* ⟨pattern⟩ ⟨ellipsis⟩) | #(⟨pattern⟩*)
| #(⟨pattern⟩* ⟨pattern⟩ ⟨ellipsis⟩) | ⟨pattern datum⟩
⟨pattern datum⟩ −→ ⟨string⟩ | ⟨character⟩
| ⟨boolean⟩
| ⟨number⟩
⟨template⟩ −→ ⟨pattern identifier⟩
| (⟨template element⟩*)
| (⟨template element⟩+ . ⟨template⟩) | #(⟨template element⟩*)
| ⟨template datum⟩
⟨template element⟩ −→ ⟨template⟩ | ⟨template⟩ ⟨ellipsis⟩
⟨template datum⟩ −→ ⟨pattern datum⟩
⟨pattern identifier⟩ −→ ⟨any identifier except …⟩ ⟨ellipsis⟩ −→ ⟨the identifier …⟩
7.1.6. Programs and definitions
⟨program⟩ −→ ⟨command or definition⟩*
⟨command or definition⟩ −→ ⟨command⟩ | ⟨definition⟩
| ⟨syntax definition⟩
| (begin ⟨command or definition⟩+) ⟨definition⟩ −→ (define ⟨variable⟩ ⟨expression⟩)
| (define (⟨variable⟩ ⟨def formals⟩) ⟨body⟩)
| (begin ⟨definition⟩*) ⟨def formals⟩ −→ ⟨variable⟩*
| ⟨variable⟩* . ⟨variable⟩ ⟨syntax definition⟩ −→
(define-syntax ⟨keyword⟩ ⟨transformer spec⟩)
7.2. Formal semantics
This section provides a formal denotational semantics for the primitive expressions of Scheme and selected built-in procedures. The concepts and notation used here are de- scribed in [29]; the notation is summarized below:
⟨…⟩ s↓k #s
s§t s†k
t → a, b ρ[x/i] x in D x|D
sequence formation
kth member of the sequence s (1-based) length of sequence s
concatenation of sequences s and t
drop the first k members of sequence s McCarthy conditional “if t then a else b” substitution “ρ with x for i”
injection of x into domain D
projection of x to domain D
The reason that expression continuations take sequences of values instead of single values is to simplify the formal treatment of procedure calls and multiple return values.
The boolean flag associated with pairs, vectors, and strings will be true for mutable objects and false for immutable ob jects.
The order of evaluation within a call is unspecified. We mimic that here by applying arbitrary permutations per- mute and unpermute, which must be inverses, to the argu- ments in a call before and after they are evaluated. This is not quite right since it suggests, incorrectly, that the order of evaluation is constant throughout a program (for any given number of arguments), but it is a closer approxima- tion to the intended semantics than a left-to-right evalua- tion would be.
The storage allocator new is implementation-dependent, but it must obey the following axiom: if new σ ∈ L, then σ (new σ | L) ↓ 2 = false.
The definition of K is omitted because an accurate defini- tion of K would complicate the semantics without being very interesting.
If P is a program in which all variables are defined before being referenced or assigned, then the meaning of P is
E[((lambda (I*) P’) ⟨undefined⟩ …)]
where I* is the sequence of variables defined in P, P′ is the sequence of expressions obtained by replacing every defini- tion in P by an assignment, ⟨undefined⟩ is an expression that evaluates to undefined, and E is the semantic function that assigns meaning to expressions.
7.2.1. Abstract syntax
E[I] = λρκ . hold (lookup ρ I)
(single(λε . ε = undefined →
wrong “undefined variable”, send ε κ))
E[(E0
λρκ . E*(permute(⟨E0⟩ § E*))
ρ
(λε* . ((λε* . applicate (ε* ↓ 1) (ε* † 1) κ)
(unpermute ε*))) E[(lambda (I*) Γ* E0)] =
λρκ . λσ .
new σ ∈ L →
send (⟨new σ | L,
λε*κ′ . #ε* = #I* →
tievals(λα* . (λρ′ . C[Γ*]ρ′(E[E0]ρ′κ′)) (extends ρ I* α*))
ε*,
wrong “wrong number of arguments”⟩
K ∈ Con I ∈ Ide E ∈ Exp
Γ ∈ Com = Exp
constants, including quotations identifiers (variables) expressions
commands
Exp−→K|I|(E0 E*)
| (lambda (I*) Γ* E0)
| (lambda (I* . I) Γ* E0)
| (lambda I Γ* E0)
|(ifE0 E1 E2)|(ifE0 E1) | (set! I E)
in E)
(update (new σ | L) unspecified σ),
7.2.2. Domain equations
α∈ L ν∈ N
T = {false, true} Q
H
R
locations natural numbers booleans symbols characters numbers
pairs
vectors
strings
E[(lambda (I* . I) Γ* E0)] = λρκ . λσ .
new σ ∈ L →
send (⟨new σ | L,
φ∈ F ε ∈ E
σ∈ S ρ∈ U θ∈ C κ∈ K
A X
miscellaneous =L×(E*→K→C) procedurevalues
= Q+H+R+Ep +Ev +Es +M+F expressed values
(update (new σ | L) unspecified σ), wrong “out of memory” σ
E[(lambda I Γ* E0)] = E[(lambda (. I) Γ* E0)] E[(if E0 E1 E2)] =
λρκ . E[E0] ρ (single (λε . truish ε → E[E1]ρκ, E [E2 ]ρκ))
E[(if E0 E1)] =
λρκ . E[E0] ρ (single (λε . truish ε → E[E1]ρκ,
send unspecified κ))
Here and elsewhere, any expressed value other than undefined
may be used in place of unspecified.
E[(set! I E)] =
λρκ . E[E] ρ (single(λε . assign (lookup ρ I)
ε
(send unspecified κ)))
E*[] = λρκ.κ⟨⟩ E*[E0 E*] =
λρκ . E[E0] ρ (single(λε0 . E*[E*] ρ (λε* . κ (⟨ε0⟩ § ε*)))) C[ ] = λρθ . θ
C[Γ0 Γ*] = λρθ . E[Γ0] ρ (λε* . C[Γ*]ρθ)
Ep =L×L×T
Ev = L* × T
Es = L* × T
M = {false, true, null, undefined, unspecified}
λε*κ′ . #ε* ≥ #I* → tievalsrest
(λα* . (λρ′ . C[Γ*]ρ′(E[E0]ρ′κ′)) (extends ρ (I* § ⟨I⟩) α*))
ε*
(#I*),
wrong “too few arguments”⟩ in E)
=L→(E×T) =Ide→L =S→A =E*→C
stores
environments
command continuations expression continuations answers
errors
7.2.3. Semantic functions
K : Con → E
E : Exp → U → K → C
E* : Exp* → U → K → C C : Com* → U → C → C
Definition of K deliberately omitted. E [K] = λρκ . send (K[K]) κ
E*)] =
κ
wrong “out of memory” σ
κ
7. Formal syntax and semantics 41
42 Revised5 Scheme
7.2.4. Auxiliary functions
lookup : U → Ide → L lookup = λρI . ρI
extends : U → Ide* → L* → U extends =
λρI*α*.#I* = 0 → ρ, extends(ρ[(α*↓1)/(I*↓1)])(I*†1)(α*†1)
list:E*→K→C list =
λε*κ . #ε* = 0 → send null κ,
list (ε* † 1)(single(λε . cons⟨ε* ↓ 1, ε⟩κ))
cons : E* → K → C cons =
twoarg (λε1ε2κσ . new σ ∈ L →
(λσ′ .newσ′ ∈L→
wrong : X → C send : E → K → C
send = λεκ . κ⟨ε⟩
[implementation-dependent]
send(⟨new σ | L,new σ′ | L,true⟩ in E)
κ
(update(new σ′ | L)ε2σ′), wrong “out of memory”σ′)
(update(new σ | L)ε1σ), wrong “out of memory”σ)
single : (E → C) → K single =
λψε*.#ε* = 1 → ψ(ε* ↓ 1),
wrong “wrong number of return values”
less : E* → K → C less =
new : S → (L + {error}) hold : L → K → C
[implementation-dependent] hold = λακσ . send (σα ↓ 1)κσ
twoarg (λε1ε2κ . (ε1 ∈ R ∧ ε2 ∈ R) →
send(ε1 | R < ε2 | R → true,false)κ,
wrong “non-numeric argument to <”)
add : E* → K → C add =
twoarg (λε1ε2κ . (ε1 ∈ R ∧ ε2 ∈ R) →
send((ε1 |R+ε2 |R) in E)κ,
wrong “non-numeric argument to +”)
car : E* → K → C car =
onearg(λεκ.ε∈Ep →hold(ε|Ep ↓1)κ,
wrong “non-pair argument to car”)
cdr : E* → K → C [similar to car]
setcar : E* → K → C setcar =
twoarg(λε1ε2κ.ε1 ∈ Ep →
(ε1 | Ep ↓ 3) → assign (ε1 | Ep ↓ 1)
assign : L → E → C → C
assign = λαεθσ . θ(update αεσ)
update : L → E → S → S update = λαεσ . σ[⟨ε, true⟩/α]
tievals : (L* → C) → E* → C tievals =
λψε*σ . #ε* = 0 → ψ⟨ ⟩σ,
new σ ∈ L → tievals (λα* . ψ(⟨new σ | L⟩ § α*))
(ε*†1)
(update(new σ | L)(ε* ↓ 1)σ), wrong “out of memory”σ
tievalsrest : (L* → C) → E* → N → C tievalsrest =
λψε*ν . list (dropfirst ε*ν)
(single(λε . tievals ψ ((takefirst ε*ν) § ⟨ε⟩)))
dropfirst = λln . n = 0 → l, dropfirst (l † 1)(n − 1)
takefirst = λln . n = 0 → ⟨ ⟩, ⟨l ↓ 1⟩ § (takefirst (l † 1)(n − 1))
truish : E → T
truish = λε . ε = false → false, true
ε2
(send unspecified κ), wrong “immutable argument to set-car!”,
wrong “non-pair argument to set-car!”)
permute : Exp* → Exp* unpermute : E* → E*
[implementation-dependent] [inverse of permute]
eqv : E* → K → C eqv =
applicate : E → E* → K → C applicate =
λεε*κ . ε ∈ F → (ε | F ↓ 2)ε*κ, wrong “bad procedure”
onearg : (E → K → C) → (E* → K → C) onearg =
λζε*κ . #ε* = 1 → ζ(ε* ↓ 1)κ,
wrong “wrong number of arguments”
twoarg : (E → E → K → C) → (E* → K → C) twoarg =
λζε*κ . #ε* = 2 → ζ(ε* ↓ 1)(ε* ↓ 2)κ,
wrong “wrong number of arguments”
twoarg (λε1ε2κ . (ε1 ∈ M ∧ ε2 ∈ M) →
send(ε1 | M = ε2 | M → true,false)κ,
(ε1 ∈ Q ∧ ε2 ∈ Q) →
send(ε1 | Q = ε2 | Q → true,false)κ,
(ε1 ∈ H ∧ ε2 ∈ H) →
send(ε1 | H = ε2 | H → true,false)κ,
(ε1 ∈ R ∧ ε2 ∈ R) →
send(ε1 | R = ε2 | R → true,false)κ,
(ε1 ∈ Ep ∧ ε2 ∈ Ep) →
send((λp1p2 .((p1 ↓1)=(p2 ↓1)∧
(p1 ↓2)=(p2 ↓2))→true, false)
(ε1 |Ep)
(ε2 |Ep)) κ,
apply : E* → K → C apply =
cwcc : E* → K → C cwcc =
[call-with-current-continuation]
(ε1 ∈Ev ∧ε2 ∈Ev)→..., (ε1 ∈ Es ∧ ε2 ∈ Es) → ..., (ε1 ∈ F∧ε2 ∈ F) →
((cond (test)) test)
((cond (test) clause1 clause2 ...)
(let ((temp test))
(if temp
temp
(cond clause1 clause2 ...))))
((cond (test result1 result2 ...))
(if test (begin result1 result2 ...)))
((cond (test result1 result2 ...)
clause1 clause2 ...)
(if test
(begin result1 result2 ...)
(cond clause1 clause2 ...)))))
(define-syntax case
(syntax-rules (else)
((case (key ...)
clauses ...)
(let ((atom-key (key ...)))
(case atom-key clauses ...)))
((case key
(else result1 result2 ...))
(begin result1 result2 ...))
((case key
((atoms ...) result1 result2 ...))
(if (memv key ’(atoms ...))
(begin result1 result2 ...)))
((case key
((atoms ...) result1 result2 ...)
clause clauses ...)
(if (memv key ’(atoms ...))
(begin result1 result2 ...)
(case key clause clauses ...)))))
(define-syntax and
(syntax-rules ()
((and) #t)
((and test) test)
((and test1 test2 ...)
(if test1 (and test2 ...) #f))))
(define-syntax or
(syntax-rules ()
((or) #f)
((or test) test)
((or test1 test2 ...)
(let ((x test1))
(if x x (or test2 ...))))))
(define-syntax let
(syntax-rules ()
((let ((name val) ...) body1 body2 ...)
((lambda (name ...) body1 body2 ...)
val ...))
((let tag ((name val) ...) body1 body2 ...)
((letrec ((tag (lambda (name ...)
body1 body2 ...)))
tag)
send((ε1 |F↓1)=(ε2 |F↓1)→true,false) κ,
send false κ)
twoarg (λε1 ε2 κ . ε1 ∈ F → valueslist ⟨ε2 ⟩(λε* . applicate ε1 ε*κ), wrong “bad procedure argument to apply”)
valueslist : E* → K → C valueslist =
onearg(λεκ.ε∈ Ep → cdr⟨ε⟩
(λε* . valueslist ε*
(λε* . car⟨ε⟩(single(λε . κ(⟨ε⟩ § ε*))))), ε = null → κ⟨ ⟩,
wrong “non-list argument to values-list”)
onearg (λεκ . ε ∈ F →
(λσ . new σ ∈ L →
applicate ε
⟨⟨new σ | L, λε*κ′ . κε*⟩ in E⟩
κ
(update (new σ | L)
unspecified
σ),
wrong “out of memory” σ),
wrong “bad procedure argument”) values : E* → K → C
cwv =
values = λε*κ . κε* cwv : E* → K → C
[call-with-values] twoarg(λε1ε2κ.applicateε1⟨⟩(λε*.applicateε2 ε*))
7.3. Derived expression types
This section gives macro definitions for the derived expres- sion types in terms of the primitive expression types (lit- eral, variable, call, lambda, if, set!). See section 6.4 for a possible definition of delay.
(define-syntax cond
(syntax-rules (else =>)
((cond (else result1 result2 …))
(begin result1 result2 …))
((cond (test => result))
(let ((temp test))
(if temp (result temp))))
((cond (test => result) clause1 clause2 …)
(let ((temp test))
(if temp
(result temp)
(cond clause1 clause2 …))))
7. Formal syntax and semantics 43
44 Revised5 Scheme val …))))
(define-syntax let*
(syntax-rules ()
((let* () body1 body2 …)
(let () body1 body2 …))
((let* ((name1 val1) (name2 val2) …)
body1 body2 …)
(let ((name1 val1))
(let* ((name2 val2) …)
body1 body2 …)))))
The following letrec macro uses the symbol
(define-syntax letrec
(syntax-rules ()
((letrec ((var1 init1) …) body …) (letrec “generate temp names”
(var1 …)
()
((var1 init1) …)
body …))
((letrec “generate temp names” ()
(temp1 …)
((var1 init1) …)
body …)
(let ((var1
(let ((temp1 init1) …)
(set! var1 temp1)
…
body …)))
((letrec “generate temp names” (x y …)
(temp …)
((var1 init1) …)
body …)
(letrec “generate temp names” (y …)
(newtemp temp …)
((var1 init1) …)
body …))))
(define-syntax begin
(syntax-rules ()
((begin exp …)
((lambda () exp …)))))
The following alternative expansion for begin does not make use of the ability to write more than one expression
in the body of a lambda expression. In any case, note that these rules apply only if the body of the begin contains no definitions.
(define-syntax begin
(syntax-rules ()
((begin exp)
exp)
((begin exp1 exp2 …)
(let ((x exp1))
(begin exp2 …)))))
The following definition of do uses a trick to expand the variable clauses. As with letrec above, an auxiliary macro would also work. The expression (if #f #f) is used to obtain an unspecific value.
(define-syntax do
(syntax-rules ()
((do ((var init step …) …)
(test expr …)
command …)
(letrec
((loop
(lambda (var …)
(if test
(begin
(if #f #f)
expr …)
(begin
command
…
(loop (do “step” var step …)
…))))))
(loop init …)))
((do “step” x)
x)
((do “step” x y)
y)))
NOTES
EXAMPLE
Language changes
This section enumerates the changes that have been made to Scheme since the “Revised4 report” [6] was published.
• The report is now a superset of the IEEE standard for Scheme [13]: implementations that conform to the report will also conform to the standard. This required the following changes:
– The empty list is now required to count as true.
– The classification of features as essential or inessential has been removed. There are now three classes of built-in procedures: primitive, li- brary, and optional. The optional procedures are load, with-input-from-file, with-output- to-file, transcript-on, transcript-off, and interaction-environment, and – and / with more than two arguments. None of these are in the IEEE standard.
– Programs are allowed to redefine built-in proce- dures. Doing so will not change the behavior of other built-in procedures.
• Port has been added to the list of disjoint types.
• The macro appendix has been removed. High-level macros are now part of the main body of the report. The rewrite rules for derived expressions have been replaced with macro definitions. There are no reserved identifiers.
• Syntax-rules now allows vector patterns.
• Multiple-value returns, eval, and dynamic-wind have
been added.
• The calls that are required to be implemented in a properly tail-recursive fashion are defined explicitly.
• ‘@’ can be used within identifiers. ‘|’ is reserved for possible future extensions.
ADDITIONAL MATERIAL
The Internet Scheme Repository at
http://www.cs.indiana.edu/scheme-repository/
contains an extensive Scheme bibliography, as well as pa- pers, programs, implementations, and other material re- lated to Scheme.
Integrate-system integrates the system
yk′ = fk(y1,y2,…,yn), k = 1,…,n
of differential equations with the method of Runge-Kutta.
The parameter system-derivative is a function that takes a system state (a vector of values for the state vari- ables y1, . . . , yn) and produces a system derivative (the val- ues y1′ , . . . , yn′ ). The parameter initial-state provides an initial system state, and h is an initial guess for the length of the integration step.
The value returned by integrate-system is an infinite stream of system states.
(define integrate-system
(lambda (system-derivative initial-state h)
(let ((next (runge-kutta-4 system-derivative h)))
(letrec ((states
(cons initial-state
(delay (map-streams next
states)))))
states))))
Runge-Kutta-4 takes a function, f, that produces a system derivative from a system state. Runge-Kutta-4 produces a function that takes a system state and produces a new system state.
(define runge-kutta-4
(lambda (f h)
(let ((*h (scale-vector h))
(*2 (scale-vector 2))
(*1/2 (scale-vector (/ 1 2)))
(*1/6 (scale-vector (/ 1 6))))
(lambda (y)
;; y is a system state (let* ((k0 (*h (f y)))
(k1 (*h (f (add-vectors y (*1/2 k0)))))
(k2 (*h (f (add-vectors y (*1/2 k1)))))
(k3 (*h (f (add-vectors y k2)))))
(add-vectors y
(*1/6 (add-vectors k0
(define elementwise
(lambda (f)
(lambda vectors
(generate-vector
(vector-length (car vectors))
(lambda (i)
(apply f
(map (lambda (v) (vector-ref v i))
vectors)))))))
(define generate-vector
(lambda (size proc)
(*2 k1)
(*2 k2)
k3))))))))
Example 45
46 Revised5 Scheme
(let ((ans (make-vector size)))
(letrec ((loop
(lambda (i)
(cond ((= i size) ans)
(else
(vector-set! ans i (proc i))
(loop (+ i 1)))))))
(loop 0)))))
(define add-vectors (elementwise +))
(define scale-vector
(lambda (s)
(elementwise (lambda (x) (* x s)))))
Map-streams is analogous to map: it applies its first argu- ment (a procedure) to all the elements of its second argu- ment (a stream).
(define map-streams
(lambda (f s)
(cons (f (head s))
(delay (map-streams f (tail s))))))
Infinite streams are implemented as pairs whose car holds the first element of the stream and whose cdr holds a promise to deliver the rest of the stream.
(define head car)
(define tail
(lambda (stream) (force (cdr stream))))
The following illustrates the use of integrate-system in integrating the system
CdvC =−iL−vC dt R
LdiL =vC dt
which models a damped oscillator.
(define damped-oscillator
(lambda (R L C)
(lambda (state)
(let ((Vc (vector-ref state 0))
(Il (vector-ref state 1)))
(vector (- 0 (+ (/ Vc (* R C)) (/ Il C)))
(/ Vc L))))))
(define the-states
(integrate-system
(damped-oscillator 10000 1000 .001)
’#(1 0)
.01))
REFERENCES
[1] Harold Abelson and Gerald Jay Sussman with Julie Sussman. Structure and Interpretation of Computer Programs, second edition. MIT Press, Cambridge, 1996.
[2] Alan Bawden and Jonathan Rees. Syntactic closures. In Proceedings of the 1988 ACM Symposium on Lisp and Functional Programming, pages 86–95.
[3] Robert G. Burger and R. Kent Dybvig. Printing floating-point numbers quickly and accurately. In Proceedings of the ACM SIGPLAN ’96 Conference on Programming Language Design and Implementa- tion, pages 108–116.
[4] William Clinger, editor. The revised revised report on Scheme, or an uncommon Lisp. MIT Artificial Intelligence Memo 848, August 1985. Also published as Computer Science Department Technical Report 174, Indiana University, June 1985.
[5] William Clinger. How to read floating point numbers accurately. In Proceedings of the ACM SIGPLAN ’90 Conference on Programming Language Design and Implementation, pages 92–101. Proceedings pub- lished as SIGPLAN Notices 25(6), June 1990.
[6] William Clinger and Jonathan Rees, editors. The revised4 report on the algorithmic language Scheme. In ACM Lisp Pointers 4(3), pages 1–55, 1991.
[7] William Clinger and Jonathan Rees. Macros that work. In Proceedings of the 1991 ACM Conference on Principles of Programming Languages, pages 155– 162.
[8] William Clinger. Proper Tail Recursion and Space Efficiency. To appear in Proceedings of the 1998 ACM Conference on Programming Language Design and Implementation, June 1998.
[9] R. Kent Dybvig, Robert Hieb, and Carl Bruggeman. Syntactic abstraction in Scheme. Lisp and Symbolic Computation 5(4):295–326, 1993.
[10] Carol Fessenden, William Clinger, Daniel P. Fried- man, and Christopher Haynes. Scheme 311 version 4 reference manual. Indiana University Computer Sci- ence Technical Report 137, February 1983. Super- seded by [11].
[11] D. Friedman, C. Haynes, E. Kohlbecker, and M. Wand. Scheme 84 interim reference manual. Indi- ana University Computer Science Technical Report 153, January 1985.
[12] IEEE Standard 754-1985. IEEE Standard for Binary Floating-Point Arithmetic. IEEE, New York, 1985.
[13] IEEE Standard 1178-1990. IEEE Standard for the Scheme Programming Language. IEEE, New York, 1991.
[14] Eugene E. Kohlbecker Jr. Syntactic Extensions in the Programming Language Lisp. PhD thesis, Indi- ana University, August 1986.
[15] Eugene E. Kohlbecker Jr., Daniel P. Friedman, Matthias Felleisen, and Bruce Duba. Hygienic macro expansion. In Proceedings of the 1986 ACM Con- ference on Lisp and Functional Programming, pages 151–161.
[16] Peter Landin. A correspondence between Algol 60 and Church’s lambda notation: Part I. Communica- tions of the ACM 8(2):89–101, February 1965.
[17] MIT Department of Electrical Engineering and Com- puter Science. Scheme manual, seventh edition. September 1984.
[18] Peter Naur et al. Revised report on the algorith- mic language Algol 60. Communications of the ACM 6(1):1–17, January 1963.
[19] Paul Penfield, Jr. Principal values and branch cuts in complex APL. In APL ’81 Conference Proceed- ings, pages 248–256. ACM SIGAPL, San Fran- cisco, September 1981. Proceedings published as APL Quote Quad 12(1), ACM, September 1981.
[20] Kent M. Pitman. The revised MacLisp manual (Sat- urday evening edition). MIT Laboratory for Com- puter Science Technical Report 295, May 1983.
[21] Jonathan A. Rees and Norman I. Adams IV. T: A dialect of Lisp or, lambda: The ultimate software tool. In Conference Record of the 1982 ACM Sym- posium on Lisp and Functional Programming, pages 114–122.
[22] Jonathan A. Rees, Norman I. Adams IV, and James R. Meehan. The T manual, fourth edition. Yale University Computer Science Department, January 1984.
[23] Jonathan Rees and William Clinger, editors. The revised3 report on the algorithmic language Scheme. In ACM SIGPLAN Notices 21(12), pages 37–79, De- cember 1986.
[24] John Reynolds. Definitional interpreters for higher order programming languages. In ACM Conference Proceedings, pages 717–740. ACM, 1972.
[25] Guy Lewis Steele Jr. and Gerald Jay Sussman. The revised report on Scheme, a dialect of Lisp. MIT Ar- tificial Intelligence Memo 452, January 1978.
[26] Guy Lewis Steele Jr. Rabbit: a compiler for Scheme. MIT Artificial Intelligence Laboratory Technical Re- port 474, May 1978.
[27] Guy Lewis Steele Jr. Common Lisp: The Language, second edition. Digital Press, Burlington MA, 1990.
[28] Gerald Jay Sussman and Guy Lewis Steele Jr. Scheme: an interpreter for extended lambda calcu- lus. MIT Artificial Intelligence Memo 349, December 1975.
[29] Joseph E. Stoy. Denotational Semantics: The Scott- Strachey Approach to Programming Language The- ory. MIT Press, Cambridge, 1977.
[30] Texas Instruments, Inc. TI Scheme Language Ref- erence Manual. Preliminary version 1.0, November 1985.
References 47
48 Revised5 Scheme
boolean?
bound 6
25; 6
caar 26
cadr 26
call 9
call by need call-with-current-continuation call-with-input-file 35 call-with-output-file 35 call-with-values 34; 8, 43 call/cc 34
car 26; 42
13
ALPHABETIC INDEX OF DEFINITIONS OF CONCEPTS, KEYWORDS, AND PROCEDURES
The principal entry for each term, procedure, or keyword is listed first, separated from the other entries by a semicolon.
case 10; 43
catch 34
cdddar 26
cddddr 26
cdr 26
ceiling 23 char->integer 29 char-alphabetic? 29 char-ci<=? 29 char-ci=? 29 char-ci>? 29 char-downcase 29 char-lower-case? 29 char-numeric? 29 char-ready? 36 char-upcase 29 char-upper-case? 29 char-whitespace? 29 char<=? 29
char=? 29
char>? 29
char? 29; 6 close-input-port 36 close-output-port 36 combination 9
comma 13
comment 5; 38
complex? 21; 19
cond 10; 15, 43
cons 26
constant 7
continuation 33
cos 23 current-input-port 35 current-output-port 35
#d 21
define 16; 14 define-syntax 17 definition 16 delay 13; 32 denominator 23 display 37
do 12; 44
dotted pair 25 dynamic-wind
#e 21; 38
!5
’ 8;26
* 22
+ 22; 5, 42 , 13;26 ,@ 13
– 22;5
-> 5
… 5; 14 / 22
;5
< 21;42 <= 21
= 21;22 => 10
> 21
>= 21
?4
‘ 13
abs 22; 24 acos 23
and 11; 43 angle 24 append 27 apply 32; 8, 43 asin 23
assoc 27 assq 27 assv 27 atan 23
#b 21; 38 backquote 13 begin 12; 16, 44 binding 6
binding construct 6
33; 8, 34, 43
34; 33
else 10
empty list 25; 6, 26 eof-object? 36 eq? 18; 10
equal? 19 equivalence predicate eqv? 17; 7, 10, 42 error 4
escape procedure eval 35; 8
even? 22
exact 17 exact->inexact 24 exact? 21 exactness 19
exp 23
expt 24
#f 25
false 6; 25 floor 23 for-each 32 force 32; 13
gcd 23 hygienic 13
17
library 3
library procedure 17 list 27 list->string 30 list->vector 31 list-ref 27 list-tail 27 list? 26
load 37
location 7
log 23
macro 13
macro keyword 13 macro transformer 13 macro use 13 magnitude 24 make-polar 24 make-rectangular 24 make-string 30 make-vector 31
map 32
max 22
member 27
memq 27
memv 27
min 22
modulo 22
mutable 7
negative? 22 newline 37
nil 25
not 25 null-environment 35 null? 26
number 19 number->string 24 number? 21; 6, 19 numerator 23 numerical types 19
#o 21; 38
ob ject 3
odd? 22 open-input-file 36 open-output-file 36 optional 3
or 11; 43 output-port? 35
pair 25
pair? 26; 6 peek-char 36 port 35 port? 6
#i 21; 38
identifier 5; 6, 28, 38
if 10; 41
imag-part 24
immutable 7
implementation restriction improper list 26
inexact 17
inexact->exact
inexact? 21
initial environment input-port? 35 integer->char 29
integer? 21; 19 interaction-environment 35
internal definition keyword 13; 38
16
lambda 9; 16, 41
lazy evaluation 13
lcm 23
length 27; 20
let 11; 12, 15, 16, 43 let* 11; 16, 44 let-syntax 14; 16 letrec 11; 16, 44 letrec-syntax 14; 16
33
24; 20 17
4; 20
Index 49
50 Revised5 Scheme
positive? 22 predicate 17 procedure call 9 procedure? 31; 6 promise 13; 32 proper tail recursion
quasiquote 13; 26 quote 8; 26 quotient 22
syntax definition 17
rational? 21; 19 rationalize 23
read 36; 26, 39 read-char 36 real-part 24
real? 21; 19 referentially transparent region 6; 10, 11, 12 remainder 22 reverse 27
round 23
13
13; 26 30; 31
scheme-report-environment 35 set! 10; 16, 41
set-car! 26
set-cdr! 26
valid indexes values 34; 9 variable 6; 5, 8, 38 vector 31 vector->list 31 vector-fill! 31
31; 20
whitespace 5 with-input-from-file 35 with-output-to-file 35 write 37; 13
write-char 37
#x 21; 39 zero? 22
setcar 42
simplest rational
sin 23
sqrt 24
string 30 string->list 30 string->number 24 string->symbol 28 string-append 30 string-ci<=? 30 string-ci=? 30 string-ci>? 30 string-copy 30 string-fill! 31 string-length 30; 20 string-ref 30 string-set! 30; 28 string<=? 30 string=? 30 string>? 30
string? 30; 6 substring 30 symbol->string symbol? 28; 6 syntactic keyword
vector-length vector-ref 31 vector-set! 31 vector? 31; 6
23
7
#t 25
tail call 7
tan 23
token 38
top level environment transcript-off 37 transcript-on 37 true 6; 10, 25 truncate 23
type 6
unbound 6; 8, 16 unquote 13; 26 unquote-splicing unspecified 4
28; 7
6; 5, 13, 38
syntax-rules
14; 17
17; 6