haskell - 如何查询统一类型到ghci？

Question

可以查询 ghci 的统一类型吗？

例如，如果我想知道 (Int -> Bool) 之间的统一类型和 (a -> Bool)我怎样才能查询到 ghci？

我试图解决的是来自 Haskell: The Craft of Functional Programming 第三版的练习 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，
塞巴斯蒂安。

Answer 1

一种方法是使用 asTypeOf :: a -> a -> a .作为函数，asTypeOf不是很有趣，但它的类型很好:它强制它的两个参数和它的返回类型统一。所以:

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

所以你可以看到这两种类型统一为 Int -> Bool .对于一个稍微有趣的例子，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，出于练习的目的，我鼓励您尝试手动统一。一旦你掌握了它，这并不难，而且是一项你会一遍又一遍地使用的技能。