parser.rkt

; I took most of this from Matt Might's:
; http://matt.might.net/articles/lexers-in-racket/
#lang racket
(require parser-tools/lex)
(require (prefix-in : parser-tools/lex-sre))

;(struct char (c) #:)

(define lex
 (lexer
    ; skip spaces:
    [#\space     (lex input-port)]
    ; skip newline:
    [#\newline   (lex input-port)]

    [#\+         'plus]
    [#\-         'minus]
    [#\*         'times]
    [#\/         'div]

    [(:: (:? #\-) (:+ (char-range #\0 #\9))) (string->number lexeme)]
    ; an actual character:
    [any-char    (string-ref lexeme 0)]))

(define ab-test-in (open-input-string "abba")) ;"1 + 2 * 3 + 4 * 1 + 3")); , aabbaa
      
(define (next-tok)
  (lex ab-test-in))

(define curtok (next-tok))

(define (accept c)
  (if (not (equal? curtok c))
      (raise 'unexpected-token)
      (begin
        (printf "Accepting ~a\n" c)
        (set! curtok (next-tok)))))

;; Example 1

(define (parse-A)
  (match curtok
    [#\a
     (begin
       (accept #\a)
       (parse-A)
       (accept #\a))]
    [#\b (parse-B)]))

(define (parse-B)
  (begin
    (accept #\b)
    (accept #\b)))