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Copyright By PowCoder代写 加微信 powcoder
How to write OurScheme (Latest modification : 02/22, 2017)
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Main code skeleton of Project 1
Print ‘Welcome to OurScheme!’
Print ‘> ‘
ReadSExp(exp);
if no error
then PrintSExp(exp);
PrintErrorMessage() ;
until (OR (user entered ‘(exit)’)
(END-OF-FILE encountered)
Print ‘Thanks for using OurScheme!’ or EOF error message
Main code skeleton of Projects 2~4
Print ‘Welcome to OurScheme!’
Print : ‘> ‘
ReadSExp( s_exp );
if no error
then result <- EvalSExp( s_exp );
PrintErrorMessage();
PrintSExp( result ) ;
else PrintErrorMessage() ;
until user has just entered LEFT_PAREN "exit" RIGHT_PAREN
EOF encountered
Print 'Thanks for using OurScheme!' or EOF error message
一、 Read in an S-expression
First, try to read in an S-expression.
terminal :
LEFT-PAREN // '('
RIGHT-PAREN // ')'
INT // e.g., '123', '+123', '-123'
STRING // "This is an example of a string."
// (strings do not extend across lines)
// OurScheme的string有C/Java的printf()的escape的概念，但只限於'\n', '\"', '\t'
// 與'\n' ； 如果'\'字元之後的字元不是'n', '"', 't', 或'\'，此(第一個)'\'字元就無
// 特殊意義(而只是一個普通字元)。
// (例： "There is an ENTER HERE>>\nSee?!”, “Use ‘\”‘ to start and close a string.”
// “OurScheme allows the use of ‘\\n’, ‘\\t’ and ‘\\\”‘ in a string.”
// “Please enter YES\NO below this line >\n”
// “You need to handle >>\\<<" "You also need to handle >>\”<<" )
DOT // '.'
FLOAT // '123.567', '123.', '.567', '+123.4', '-.123'
NIL // 'nil' or '#f', but not 'NIL' nor 'nIL'
T // 't' or '#t', but not 'T' nor '#T'
QUOTE // '
SYMBOL // a consecutive sequence of printable characters
// that are not numbers,
// and do not contain '(', ')',
// single-quote, double-quote and white-spaces ;
// Symbols are case-sensitive
// (i.e., uppercase and lowercase are different);
With the exception of strings, token are separated by the following "separators" :
(a) one or more white-spaces
(b) '(' (note : '(' is a token by itself)
(c) ')' (note : ')' is a token by itself)
(d) the single-quote character (') (note : this is a token by itself)
(e) the double-quote character (") (note : this starts a STRING)
(f) semi-colon (;) (note : this is the start of a line comment)
Examples :
'3.25' is a FLOAT.
'3.25a' is a SYMBOL.
'a.b' is a SYMBOL.
'#f' is NIL
'#fa' (alternatively, 'a#f') is a SYMBOL.
'.' can mean several things :
it is either part of a FLOAT or part of a SYMBOL or a DOT.
It means a DOT only when it "stands alone".
'#' can also mean two things :
it is either part of NIL (or T) or part of a SYMBOL.
It is part of NIL (or T) only when it is '#t' or '#f' that
"stand alone".
| LEFT-PAREN
RIGHT-PAREN
| QUOTE
| NIL | T | LEFT-PAREN RIGHT-PAREN
Once the attempt to read in an S-expression fails, the line containing the error-token is ignored. Start to read in an S-expression from the next input line.
> (t . nil . (1 2 3))
ERROR (unexpected token) : ‘)’ expected when token at Line 1 Column 10 is >>.<< > (12 ( . 3))
ERROR (unexpected token) : atom or ‘(‘ expected when token at Line 1 Column 10 is >>.<< > ERROR (unexpected token) : atom or ‘(‘ expected when token at Line 1 Column 1 is >>)<< > (1 2 3) )
ERROR (attempt to apply non-function) : 1
> ERROR (unexpected token) : atom or ‘(‘ expected when token at Line 1 Column 2 is >>)<< > ‘(1 2 3) )
> ERROR (unexpected token) : atom or ‘(‘ expected when token at Line 1 Column 2 is >>)<< > ‘(1 2 3) ‘(1 2 . 3 4)
> ERROR (unexpected token) : ‘)’ expected when token at Line 1 Column 12 is >>4<< > ‘(1 2 3) ‘(1 2 .
ERROR (unexpected token) : ‘)’ expected when token at Line 4 Column 3 is >>4<<
二、 Always check the syntax of the user’s input; Must make sure that it is an
S-expression before evaluating it.
User input 可能會有的四種syntax error的相關message(的範例)如下：
ERROR (unexpected token) : atom or '(' expected when token at Line X Column Y is >>…<<
ERROR (unexpected token) : ')' expected when token at Line X Column Y is >>…<<
ERROR (no closing quote) : END-OF-LINE encountered at Line X Column Y
ERROR (no more input) : END-OF-FILE encountered
三、 The part of eval() concerning error messages : // Note : once an error occurs,
// the call to eval() is over
if what is being evaluated is an atom but not a symbol
return that atom
else if what is being evaluated is a symbol
check whether it is bound to an S-expression or an internal function
if unbound
ERROR (unbound symbol) : abc
return that S-expression or internal function (i.e., its binding)
else // what is being evaluated is (...) ; we call it the main S-expression below
// this (...) cannot be nil (nil is an atom)
if (...) is not a (pure) list
ERROR (non-list) : (...) // (...)要pretty print
else if first argument of (...) is an atom ☆, which is not a symbol
ERROR (attempt to apply non-function) : ☆
else if first argument of (...) is a symbol SYM
check whether SYM is the name of a function (i.e., check whether 「SYM has a
binding, and that binding is an internal function」)
if SYM is the name of a known function
if the current level is not the top level, and SYM is 'clean-environment' or
or 'define' or 'exit'
ERROR (level of CLEAN-ENVIRONMENT) / ERROR (level of DEFINE) / ERROR (level of EXIT)
else if SYM is 'define' or 'set!' or 'let' or 'cond' or 'lambda'
check the format of this expression // 注意：此時尚未check num-of-arg
// (define symbol // 注意：只能宣告或設定 非primitive的symbol (這是final decision!)
// S-expression
// (define ( one-or-more-symbols )
// one-or-more-S-expressions
// (set! symbol
// S-expression
// (lambda (zero-or-more-symbols)
// one-or-more-S-expressions
// (let (zero-or-more-PAIRs)
// one-or-more-S-expressions
// (cond one-or-more-AT-LEAST-DOUBLETONs
// where PAIR df= ( symbol S-expression )
// AT-LEAST-DOUBLETON df= a list of two or more S-expressions
if format error (包括attempting to redefine system primitive)
ERROR (COND format) : ERROR (DEFINE format) : ERROR (SET! format) : ERROR (LET format) : ERROR (LAMBDA format) : evaluate ( … ) return the evaluated result (and exit this call to eval()) else if SYM is ‘if’ or ‘and’ or ‘or’ ERROR (incorrect number of arguments) : if evaluate ( … ) return the evaluated result (and exit this call to eval()) else // SYM is a known function name ‘abc’, which is neither // ‘define’ nor ‘let’ nor ‘cond’ nor ‘lambda’ ERROR (incorrect number of arguments) : abc else // SYM is ‘abc’, which is not the name of a known function ERROR (attempt to apply non-function) : ☆ // ☆ is the binding of abc else // the first argument of ( … ) is ( 。。。 ), i.e., it is ( ( 。。。 ) …… ) evaluate ( 。。。 ) // if any error occurs during the evaluation of ( 。。。 ), we just output an // an appropriate error message, and we will not proceed any further if no error occurs during the evaluation of ( 。。。 ) if the evaluated result (of ( 。。。 )) is an internal function check whether the number of arguments is correct if num-of-arguments is NOT correct ERROR (incorrect number of arguments) : name-of-the-function ERROR (incorrect number of arguments) : lambda expression // in the case of nameless functions else // the evaluated result (of ( 。。。 )) is not an internal function ERROR (attempt to apply non-function) : ☆ // ☆ is the evaluated result end of 「else the first argument of ( … ) is ( 。。。 )」 eval the second argument S2 of (the main S-expression) ( … ) if the type of the evaluated result is not correct ERROR (xxx with incorrect argument type) : the-evaluated-result // xxx must be the name of some primitive function! if no error eval the third argument S3 of (the main S-expression) ( … ) if the type of the evaluated result is not correct ERROR (xxx with incorrect argument type) : the-evaluated-result if no error return the evaluated result ERROR (no return result) : name-of-this-function ERROR (no return result) : lambda expression // if there is such a case … end // else what is being evaluated is (…) ; we call it the main S-expression 1. error message之「其他」 如果你的系統碰到一個error、而以上eval的algorithm中對此error「該有何error message」並沒有規範(這有點像是if-then-else-if-then-…-else-if-then-else中的最後那個「else」)，你就output ERROR : aaa 其中aaa是user input中「出問題的那個「被evaluate的function」的first argument」。 當你依照以上eval的algorithm來evaluate an S-expression時，你會不斷的要evaluate a function，一旦這種「project並未規範的error」發生，「當時」那個被evaluate的function的first argument就是這裡所謂的aaa。 // 「project未規範」 df= project中(與test data中)未提到這種error，但明明就是個error // OR // project中有提到這種error，但沒說error message應該是啥 2. Some examples of error messages > (car nil) ERROR (car with incorrect argument type) : nil > (define (f a) (cons a a)) ERROR (incorrect number of arguments) : f > (define (ff a) (g a a)) ff defined > (define (g a) (cons a a)) ERROR (incorrect number of arguments) : g > (define (f a) (cons a a a)) ERROR (incorrect number of arguments) : cons > (CONS 3 4) ERROR (unbound symbol) : CONS > (cons hello 4) ERROR (unbound symbol) : hello ERROR (unbound symbol) : hello > (CONS hello there) ERROR (unbound symbol) : CONS > (cons 1 2 3) ERROR (incorrect number of arguments) : cons ERROR (attempt to apply non-function) : 3 ERROR (attempt to apply non-function) : 4321 > (define a 5) ERROR (attempt to apply non-function) : 5 > (* 3 “Hi”) ERROR (* with incorrect argument type) : “Hi” > (string>? 15 “hi”) ERROR (string>? with incorrect argument type) : 15 > (+ 15 “hi”) ERROR (+ with incorrect argument type) : “hi” > (string>? “hi” “there” a) ERROR (string>? with incorrect argument type) : 5 > (string>? “hi” “there” about) ERROR (unbound symbol) : about > (cond ((> 3 4) ‘bad) ((> 4 5) ‘bad) ERROR (no return value) : ( cond > (cond ((> y 4) ‘bad) ((> 4 3) ‘good) ERROR (unbound symbol) : y Lisp and Scheme 堅持一個概念： 沒有「value」！ 只有「binding」！ 沒有「symbol的value」這回事！ 只有「symbol的binding」！ ＊ Symbol的binding可能是一個S-expression (which is basically a structure of symbols)，也可能是一個(所謂的)internal function。 ＊ Internal functions有事先system define好的，也有user define的。 ＊ evaluate 一個「非symbol的atom」 的結果 是 那個atom ＊ evaluate 一個symbol 的結果 是 那個symbol的binding ＊ evaluate 一個list 的結果 是 apply 「evaluate此list的first argument 所得的結果」(which is supposedly an internal function) 於 「evaluate此list的其他 arguments 所得的結果」 經由使用某些system defined 的”東東” (如’define’)，我們可以改變symbol的binding。 但 我們能改變「原先system已define好的symbol」的binding嗎？ 例： how about these？ > (define define 3) > (define exit 3) > (let ((cons car)) (cons ‘(1 2))) Petite Chez Scheme 允許如此！ OurScheme要不要？ 答案： 我們 不允許 改變”primitive symbol”的binding！ // 有人曾問這行不行： (define 3 4) // 答案固然是不行，但原因是： ‘define’只能改變「symbol」的binding (而3不是個symbol) // 也有人曾問這行不行： (define nil 4) // 答案固然是不行，但原因是： ‘nil’不是「symbol」(numbers, strings, #f, nil都不是symbols) But note that below is OK. > (define myCons cons) myCons defined # # > (myCons 3 5) > (define a (myCons car cdr)) ( # # > ((car a) ‘(1 2 3)) > ((cdr a) ‘(1 2 3)) Explanation : # “Hi”, (1 2 3) and (1 (2 3) “Hi), and should be treated in the same way. (But of course, # these “others” cannot do. But that is a different story.) Below, the word ‘symbol’ should be taken to mean : a symbol that is not a primitive symbol (i.e., it is not a pre-defined symbol) ＊ ‘car’ expects its argument to be a cons-cell. ＊ ‘cdr’ expects its argument to be a cons-cell. ＊ ‘quote’ expects its argument to be an S-expression. ＊ ‘define’ expects that either its first argument is a symbol or its first argument is a list of one or more symbols. ＊ ‘lambda’ expects that its first argument is a list of zero or more symbols. ＊ ‘let’ expects its first argument to be a list of one or more pairs, with the first element of each pair being a symbol. ＊ ‘+’, ‘-‘,’*’,’/’ expect their arguments to be numbers. ＊ ‘>’, ‘>=’,’<','<=','=' expect their arguments to be numbers.
＊ 'string-append' and 'string>?’ expect their arguments to be strings. ＊ ‘set!’, ‘set-car!’ and ‘set-cdr!’ expect their first argument to be a symbol. ＊ ‘display-string’ expects its argument to be a string. ＊ In all other cases, S-expressions or internal functions are expected as arguments. 程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com
check whether the number of arguments is correct
if number of arguments is NOT correct
check whether the number of arguments is correct
if number of arguments is NOT correct
ERROR (unbound symbol) : abc
check whether the evaluated result (of ( 。。。 )) is an internal function
apply the binding of the first argument (an internal function) to S2-eval-result,
S3-eval-result, …
if no error
if there is an evaluated result to be returned
3.value and binding
4. Expected argument type