如何在ghci中查询统一类型？

Question

是否可以查询ghci的统一类型？

例如，如果我想知道 (Int -> Bool) 和 (a -> Bool) 之间的统一类型，我如何查询ghci呢？

我尝试解决的问题是第三版函数式编程的Haskell实践中的练习13.23。

How can you use the Haskell system to check whether two type expressions are unifiable, and if so what is their unification? Hint: you can make dummy definitions in Haskell in which the defined value, zircon say, is equated with itself:

zircon = zircon

Values defined like this can be declared to have any type you wish.

谢谢，
Sebastián。

- Fof

Answer 1

一种方法是使用asTypeOf:: a -> a -> a。作为一个函数，asTypeOf并不是很有趣，但它的类型很好：它强制其两个参数和返回类型统一。所以：

> :t asTypeOf (undefined :: Int -> Bool) (undefined :: a -> Bool)
asTypeOf (undefined :: Int -> Bool) (undefined :: a -> Bool)
  :: Int -> Bool

您可以看到这两种类型合并为 Int -> Bool。对于一个稍微有趣一点的例子，让我们统一 Maybe a 和 f (Bool, c):

> :t asTypeOf (undefined :: Maybe a) (undefined :: f (Bool, c))
asTypeOf (undefined :: Maybe a) (undefined :: f (Bool, c))
  :: Maybe (Bool, c)

另一方面，为了练习，我鼓励你手动尝试统一操作。一旦你掌握了技巧，它并不难，并且是一项你将反复使用的技能。

- Daniel Wagner

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还可以使用列表构造器 :t [undefined :: Maybe a, undefined :: f (Bool, c)]。 - max taldykin