struct - Racket 契约(Contract)和结构问题

Question

我正在为人工智能类(class)学习 Racket。对于第一个项目，老师给了我们一个包含契约(Contract)和单元测试的文件，我们要编写使其运行所需的功能。我刚刚创建了我需要的功能的 stub ，并满足了除一个之外的所有契约(Contract):

[start-state (and/c state? (not/c state-game-over?))]

我声明的函数现在看起来像这样:

(define (start-state)
  (state '() start-tiles 0)
)

状态结构是老师给出的:

(struct state (played unplayed passes) #:prefab)

有契约(Contract):

[struct state ((played (listof (and/c tile? tile-at-origin?)))
             (unplayed (listof (and/c tile? tile-on-board?)))
             (passes pass-count?))]

这会因错误而崩溃:

start-state: broke its contract
promised: (and/c state? (not/c state-game-over?))
produced: #<procedure:start-state>
which isn't: state?
in: (and/c state? (not/c state-game-over?))
contract from: 

我相信我的起始状态过程创建并返回一个状态结构，但显然它返回自身并违反了契约(Contract)。如何返回结构而不是过程？

Answer 1

它看起来像 start-state不应该是一个过程，而是一个值。也就是说，你需要做

(define start-state (start ...))

而不是

(define (start-state) ...)