球拍参数合同错误

Question

我已经开始学习合约了，我有这样的程序：

(define/contract (foldr-map f a xs)
  foldr-map/c
  (define (it a xs ys)
    (if (null? xs)
        (cons ys a)
        (let* [(p (it a (cdr xs) ys))
              (fc (f (car xs) (cdr p)))]
          (cons (cons (car fc) (car p)) (cdr fc)))))

  (it a xs null))

(foldr-map (lambda (x a) (cons a (+ a x))) 0 `(1 2 3))

我有 flodr-map/c 合同定义为：


(define foldr-map/c
  (parametric->/c [x a] (->
                         (-> x a (cons/c a number?))
                         a
                         (listof x)
                         (cons/c (listof a) a))))

但是我看到这样的错误：

foldr-map: broke its own contract
  promised: a
  produced: 3
  in: the 2nd argument of
      the 1st argument of
      (parametric->/c
       (x a)
       (->
        (-> x a (cons/c a number?))
        a
        (listof x)
        (cons/c (listof a) a)))
  contract from: (function foldr-map)
  blaming: (function foldr-map)
   (assuming the contract is correct)

我知道程序运行正常，所以合同一定是错的。该过程采用函数参数 f。

合约有2个参数：

x 是列表 xs
a 是一个累加器

当我将合同更改为：

(define foldr-map/c
  (parametric->/c [x a] (->
                         (-> x number? (cons/c number? number?))
                         a
                         (listof x)
                         (cons/c (listof a) a))))

我收到这样的错误：

foldr-map: broke its own contract
  promised: number?
  produced: #<a>
  in: the 2nd argument of
      the 1st argument of
      (parametric->/c
       (x a)
       (->
        (-> x number? (cons/c number? number?))
        a
        (listof x)
        (cons/c (listof a) a)))
  contract from: (function foldr-map)
  blaming: (function foldr-map)
   (assuming the contract is correct)

所以在这一点上我迷路了。

Answer 1

我找到了解决方案：

(define foldr-map/c
  (parametric->/c [x a] (->
                         (-> x a (cons/c a a))
                         a
                         (listof x)
                         (cons/c (listof a) a))))

这是有道理的，而且看起来很明显，但花了相当多的调试时间才弄明白。

球拍参数合同错误

Racket parametric contracts errors

lisp

scheme

functional-programming

racket