Simple parser combinators in Scheme

parsec.scm

(define (return v) (lambda (s ks kf) (ks v s)))
(define fail (lambda (s ks kf) (kf)))

; >>=
(define (bind a f)
  (lambda (s ks kf)
    (a s
       (lambda (av s1) ((f av) s1 ks kf))
       kf)))

(define (mplus a b)
  (lambda (s ks kf)
    (a s ks
       (lambda () (b s ks kf)))))

(define (psym pred)
  (lambda (s ks kf)
    (if (null? s)
      (kf)
      (if (pred (car s))
        (ks (car s) (cdr s))
        (kf)))))

(define (char a) (psym (lambda (c) (eq? a c))))

(define (many p)
  (mplus
    (bind
      p (lambda (pv)
          (bind (many p) (lambda (pvs) (return (cons pv pvs))))))
    (return '())))

(define (run-parser p str)
  (p (string->list str)
     (lambda (v s) (format #t "Parser ran successfully. Result is ~a, remainder is ~a.~%" v (list->string s)))
     (lambda () (format #t "Parser failed.~%"))))