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This file contains definitions for the OCaml type THING, and the OCaml module
SCANNER, both of which are needed to write the module PARSER for Project 2.

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(* THING. Types of the usual Lisp objects. *)

Closure of thing * thing * environment |
Cons of thing * thing |
Number of int |
Primitive of (thing -> environment -> thing) |
Symbol of string
environment = (string * thing) list ;;

(* SCANNER. Lexical scanner for Lisp from Lab 9. It also ignores comments. *)

module Scanner =

(* TOKEN. A token for an expression in a subset of Lisp. *)

type token =
CloseParenToken |
EndToken |
NumberToken of int |
OpenParenToken |
SymbolToken of string ;;

(* MAKE SCANNER. Return a version of the scanner function NEXT TOKEN that reads
TOKENs from a file whose pathname is the string PATH. INPUT is a channel
connected to the file. CH holds the most recently read CHAR from INPUT. *)

let makeScanner path =
let input = open_in path
in let ch = ref ‘ ‘

(* NEXT CHAR. Advance CH to the next CHAR from INPUT. If there is no next CHAR,
then set CH to ‘\000’. We use this CHAR to represent the end of a file. We’d
like to give this CHAR a name, but then we couldn’t MATCH on that name. *)

let nextChar () =
try ch := input_char input
with End_of_file ->
ch := ‘\000’

(* NEXT CLOSE PAREN TOKEN. Read a CLOSE PAREN TOKEN. *)

let nextCloseParenToken () =
nextChar () ;
CloseParenToken

(* NEXT COMMENT. Skip a comment. It starts with a ‘;’ and ends with a newline
‘\n’ or an end of file ‘\000’. We skip the ‘\n’, but not the ‘\000’. *)

let rec nextComment () =
match ! ch
with ‘\000’ ->
nextChar () |
nextChar () ;
nextComment ()

(* NEXT END TOKEN. Read an END TOKEN. We don’t skip a CHAR because there are no
more CHARs to skip. *)

let nextEndToken () =

(* NEXT NUMBER TOKEN. Read a NUMBER TOKEN that starts with PREFIX. *)

let nextNumberToken prefix =
let rec nextNumbering chars =
match ! ch
with ‘\000’ | ‘\n’ | ‘ ‘ | ‘(‘ | ‘)’ ->
NumberToken (int_of_string chars) |
let otherChars = Char.escaped ! ch
in nextChar () ;
nextNumbering (chars ^ otherChars)
in nextNumbering prefix

(* NEXT OPEN PAREN TOKEN. Read an OPEN PAREN TOKEN. *)

let nextOpenParenToken () =
nextChar () ;
OpenParenToken

(* NEXT SYMBOL TOKEN. Read a SYMBOL TOKEN that starts with PREFIX. *)

let nextSymbolToken prefix =
let rec nextSymboling chars =
match ! ch
with ‘\000’ | ‘\n’ | ‘ ‘ | ‘(‘ | ‘)’ ->
SymbolToken chars |
let otherChars = Char.escaped ! ch
in nextChar () ;
nextSymboling (chars ^ otherChars)
in nextSymboling prefix

(* NEXT NUMBER OR SYMBOL TOKEN. We’ve just read a ‘-‘, but we don’t know yet if
it starts a NUMBER TOKEN or a SYMBOL token. Skip the ‘-‘. If we see a digit,
then it starts a NUMBER TOKEN, otherwise it starts a SYMBOL TOKEN. *)

let nextNumberOrSymbolToken () =
nextChar () ;
match ! ch
with ‘0’ | ‘1’ | ‘2’ | ‘3’ | ‘4’ | ‘5’ | ‘6’ | ‘7’ | ‘8’ | ‘9’ ->
nextNumberToken “-” |
nextSymbolToken “-”

(* NEXT TOKEN. Look at CH to tell what TOKEN is coming next. Dispatch to the
function that will read that TOKEN and return it. *)

let rec nextToken () =
match ! ch
with ‘\000’ ->
nextEndToken () |
‘ ‘ | ‘\n’ ->
nextChar () ;
nextToken () |
nextOpenParenToken () |
nextCloseParenToken () |
nextComment () ;
nextToken () |
nextNumberOrSymbolToken () |
‘0’ | ‘1’ | ‘2’ | ‘3’ | ‘4’ | ‘5’ | ‘6’ | ‘7’ | ‘8’ | ‘9’ ->
nextNumberToken “” |
nextSymbolToken “”

(* Lost? This is MAKE SCANNER’s body. Initialize CH by reading the NEXT CHAR,
and return (but do not call!) NEXT TOKEN. *)

in nextChar () ;
nextToken ;;

*** YOUR CODE GOES HERE! ***

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