Simplest Contract System

contract.rkt

#lang racket

;; A Contract is one of:
;; - Any -> Boolean
;; - Signature

;; A Signature is a (signature Contract Contract)
(struct signature (domain range))


(define (assert-prop p v)
  (if (p v) v (error "contract failed")))

(define (assert-signature c v)
  (lambda (x)
    (assert (signature-range c)
            (v (assert (signature-domain c) x)))))

(define (assert c v)
  (match c
    [(? signature?) (assert-signature c v)]
    [(? procedure?) (assert-prop c v)]))