Function selection based on pre-conditions in Chez Scheme (very nice exercise using call/cc)

pre-conditions.ss

(define (pre-condition assertion) 'noop)

(define (try . procs)
  (let loop ([procs procs])
    (define proc (car procs))
    (define (make-pre-condition k)
      (lambda (assertion)
        (when (not assertion)
          (k (lambda () (loop (cdr procs)))))))
    (define (exec k)
      (fluid-let ([pre-condition (make-pre-condition k)])
        (let ([v (proc)])
          (lambda () v))))
    (define rslt (call/cc exec))
    (rslt)))

(define (a) 1)
(define (b) (pre-condition #f) 2)
(define (c) 3)

(printf "~s\n" (try a b c))
(printf "~s\n" (try b c))
(printf "~s\n" (try c))
(printf "~s\n" (try a))