McCarthy's `amb` without mutable state.

amb.rkt

(define-values (success-tag fail-tag)
  (values (make-continuation-prompt-tag 'success)
          (make-continuation-prompt-tag 'fail)))

(define-syntax amb
  (syntax-rules ()
    [(_ alt ...)
     (call/comp
      (λ (k)
        (abort
         (call/prompt
          (λ ()
            (call/prompt
             (λ ()
               (let ([x (k alt)])
                 (abort/cc success-tag (λ () x))))
             fail-tag)
            ...
            (if (continuation-prompt-available? fail-tag)
                (abort/cc fail-tag void)
                (error "amb tree exhausted")))
          success-tag)))
      (if (continuation-prompt-available? fail-tag)
          fail-tag
          (default-continuation-prompt-tag)))]))