球拍：采取：违反合同

Question

我在 linux 上使用 racket v6.5 repl 并尝试运行来自流教程 https://docs.racket-lang.org/functional-data-structures/streams.html.

的 take 函数示例

然而，与预期相反

> (take 3 (stream 1 2 3 4 5 6))
- : (Rec
 g1827317
 (U Null
    (Boxof (U (Pairof Integer g1827317) (-> (Pairof Integer g1827317))))))
'#&#<procedure:.../pfds/stream.rkt:41:7>

我明白了

> (take 3 (stream 1 2 3 4 5 6))
take: contract violation
expected: exact-nonnegative-integer?
given: #<stream>
argument position: 2nd
other arguments...:
 3
context...:
 /usr/share/racket/collects/racket/list.rkt:151:0: take
 /usr/share/racket/collects/racket/private/misc.rkt:87:7
>

Answer 1

要使用该流库，您需要像这样要求 pfds/stream：

(require pfds/stream)

完成后，take and stream 的正确版本将可用：

> (require typed/racket)
> (require pfds/stream)
> (take 3 (stream 1 2 3 4 5 6))
- : (Rec
     anonymous-module1715254
     (U Null
        (Boxof
         (U (Pairof Integer anonymous-module1715254)
            (-> (Pairof Integer anonymous-module1715254))))))
'#&#<procedure:.../pfds/stream.rkt:41:7>

球拍：采取：违反合同

Racket: take: contract violation

lisp

functional-programming

stream

racket