A simple brainfuck interpreter in Scheme.

brainfuck.rkt

#lang racket

(define-syntax incf
  (syntax-rules ()
    ((_ x) (begin (set! x (+ x 1)) x))
    ((_ x i) (begin (set! x (+ x i)) x))))

(define-syntax decf
  (syntax-rules ()
    ((_ x) (begin (set! x (- x 1)) x))
    ((_ x i) (begin (set! x (- x i)) x))))

(define (interpreter instruction)
  (let ((i 0) (ptr 0) (mem (make-vector 20 0)) (end (string-length instruction)))
    (do ((i 0 (incf i)))
           ((= i end))
           (let ((current-instruction (string-ref instruction i)))
             (case current-instruction
               ((#\>) (incf ptr))
               ((#\<) (decf ptr))
               ((#\-) (vector-set! mem ptr (- (vector-ref mem ptr) 1)))
               ((#\+) (vector-set! mem ptr (+ (vector-ref mem ptr) 1)))
               ((#\.) (write-byte (vector-ref mem ptr)))
               ((#\,) (vector-set! mem ptr (read-byte)))
               ((#\[) (if (zero? (vector-ref mem ptr))
                           (let ((loop 1))
                             (do()
                               ((zero? loop))
                               (incf i)
                               (let ((current-instruction (string-ref instruction i)))
                                  (if (eq? #\] current-instruction) (decf loop) '())
                                  (if (eq? #\[ current-instruction) (incf loop) '()))))
                           '() ))
               ((#\]) (if (zero? (vector-ref mem ptr))
                           '()
                           (let ((loop 1))
                             (do()
                               ((zero? loop))
                               (decf i)
                                (let ((current-instruction (string-ref instruction i)))
                                  (if (eq? #\[ current-instruction) (decf loop) '())
                                  (if (eq? #\] current-instruction) (incf loop) '()))))))               
               (else 'fail)
               )
             )
      ))
 )

(interpreter "++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.")