在haskell中自动转换类型 [英] Auto convert type in haskell

问题描述

我写了一些有用的函数来做逻辑运算。它看起来像（a和b或c）`belongs` x 。

I've written some helpful functions to do logical operation. It looks like (a and b or c) `belongs` x.

感谢 Num ， IsList 和 OverloadedLists ，我可以将它用于整数和列表。 / p>

Thanks to Num, IsList and OverloadedLists, I can have it for integers and lists.

λ> (1 && 2 || 3) `belongs` [2]
False
λ> (1 && 2 || 3) `belongs` [1, 2]
True
λ> (1 && 2 || 3) `belongs` [3]
True

λ> :set -XOverloadedLists
λ> ([1, 2] && [2, 3] || [3, 4]) `contains` 1
False
λ> ([1, 2] && [2, 3] || [3, 4]) `contains` 2
True

我这样做：

I do it this way:

newtype BoolLike a = BoolLike ((a -> Bool) -> Bool)

(&&) :: BoolLike a -> BoolLike a -> BoolLike a
BoolLike f && BoolLike g = BoolLike $ \k -> f k P.&& g k
infixr 3 &&

toBoolLike :: a -> BoolLike a
toBoolLike x = BoolLike $ \k -> k x

belongs :: Eq a => BoolLike a -> [a] -> Bool
belongs (BoolLike f) xs = f (\x -> x `elem` xs)

contains :: Eq a => BoolLike [a] -> a -> Bool
contains (BoolLike f) x = f (\xs -> x `elem` xs)

instance Num a => Num (BoolLike a) where
  fromInteger = toBoolLike . fromInteger

我要做的是适用于任何类型，无需手动转换 a 到 BoolLike a 。

What I am going to do is make it suit for any types, without manually convert a to BoolLike a.

我该如何实现它？

第一次尝试：

First try:

class IsBool a where
  type BoolItem a
  toBool :: a -> BoolItem a

instance IsBool (BoolLike a) where
  type BoolItem (BoolLike a) = BoolLike a
  toBool = id

instance IsBool a where
  type BoolItem a = BoolLike a
  toBool = toBoolLike

失败：

Conflicting family instance declarations:
  BoolItem (BoolLike a) -- Defined at BoolLike.hs:54:8
  BoolItem a -- Defined at BoolLike.hs:58:8

推荐答案

你可以试试这个 b

You could try this

type family BoolItem a where
    BoolItem (BoolLike a) = BoolLike a
    BoolItem a = BoolLike a

class IsBool a where
  toBool :: a -> BoolItem a

instance IsBool (BoolLike a) where
  toBool = id

instance (BoolItem a ~ BoolLike a) => IsBool a where
  toBool = toBoolLike

通过将类型族移出课程，您可以为所有类型定义它。然后剩下的就是限制其中一个实例。别忘了你还需要做一个 OVERLAPPABLE

By moving the type family out of the class you can define it for all types. Then all that remains is restricting one of the instances. Don't forget you'll also need to make that one OVERLAPPABLE

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