haskell - 是否无法计算 Enum 类型的 maxBound 和 minBound 之间的差异？

Question

我是否可以得出无法计算 maxBound - minBound 的结论？在 Haskell 中获取任意 Enum和 Bounded类型？还是我错过了一些技巧/黑客？这就是我所拥有的，这显然行不通:

difference :: (Enum a, Bounded a) => Int
difference = fromEnum maxBound - fromEnum minBound

错误:

Foo.hs:37:1:
    Ambiguous constraint `Enum a'
      At least one of the forall'd type variables mentioned by the constraint
      must be reachable from the type after the '=>'
    In the type signature for `difference': difference :: (Enum a, Bounded a) => Int

Foo.hs:37:1:
    Ambiguous constraint `Bounded a'
      At least one of the forall'd type variables mentioned by the constraint
      must be reachable from the type after the '=>'
   In the type signature for `difference': difference :: (Enum a, Bounded a) => Int

我明白为什么会出现这个错误——其中没有类型为 a 的实际术语。 ，所以它无法弄清楚 a 是什么是。问题是是否有办法解决这个问题。

Answer 1

difference :: (Enum a, Bounded a) => a -> Int
difference x = fromEnum (maxBound `asTypeOf` x) 
             - fromEnum (minBound `asTypeOf` x)

将其称为例如 difference (undefined :: Char) .

但请注意，这对于某些类型(最明显的是 Int )会溢出，因此请改用 Integer结果:

difference :: (Enum a, Bounded a) => a -> Integer
difference x = toEnum (fromEnum (maxBound `asTypeOf` x)) 
             - toEnum (fromEnum (minBound `asTypeOf` x))