Question

我有这段代码：

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE DefaultSignatures #-}

module Study where

class C a where
    type T a = r | r -> a
    pred :: T a -> Bool

    default pred :: T a ~ [a] => T a -> Bool
    pred = not . null

instance C Integer where
    type T Integer = [Integer]

它的工作原理如下：

λ Study.pred [1,2,3]
True
λ Study.pred ([ ] :: [Integer])
False

我想将简单的实例定义简化为：

instance C Integer

- 除非我特别希望与模式不同。

我计划拥有的大多数实例应该是默认的，T a ~ [a]，但有些确实需要它们拥有T类型。我不愿意容忍许多相同的琐碎定义，如给定的一。可以做些什么？

Answer 1

您只需添加默认类型实例化：

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE DefaultSignatures #-}

module Study where

data One a = One a

class C a where
    type T a = r | r -> a
    -- This looks a bit strange,
    -- because it looks like T is defined twice
    -- but that's not actually the case
    type T a = [a]
    pred :: T a -> Bool

    default pred :: T a ~ [a] => T a -> Bool
    pred = not . null

instance C Integer

instance C Char where
  type T Char = One Char
  pred = const True

*Study> :t undefined :: T Integer
undefined :: T Integer :: [Integer]
*Study> :t undefined :: T Char
undefined :: T Char :: One Char

我可以从具有关联类型系列的类的实例中删除类型实例化吗？

1 个答案: