使用关联类型族时推断类型类约束
我知道你可以在关联的类型族和数据族上添加约束条件。 它所做的是强制对所有类的实例进行约束。
但我无法弄清楚如何在实例派生或函数声明中推断这些约束。 例如，这段代码无法输入check：
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}
import Data.Proxy ( Proxy )
class Eq (FooT a) => Foo a where
type FooT a :: *
-- Can't infer it in an instance derivation
data CantInferEq a = CantInferEq (FooT a) deriving Eq
-- Also can't infer it in a function declaration.
-- The Proxy is there to avoid non-injectivity issues.
cantInferEq :: Proxy a -> FooT a -> FooT a -> Bool
cantInferEq _ x y = x == y
错误消息是：
Test.hs:11:52: No instance for (Eq (FooT a)) …
arising from the first field of ‘CantInferEq’ (type ‘FooT a’)
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
When deriving the instance for (Eq (CantInferEq a))
Test.hs:16:23: No instance for (Eq (FooT a)) arising from a use of ‘==’ …
In the expression: x == y
In an equation for ‘cantInferEq’: cantInferEq _ x y = x == y
Compilation failed.
这里发生了什么？ 有没有解决方法来获得我想要的行为？
问题的症结在于只给出了一个FooT a ，你无法从中提取Eq实例字典。
解决方法是在你的类类需求中明确的，从而有一个地方通过Eq字典：
{-# LANGUAGE StandaloneDeriving, UndecidableInstances #-}
data CantInferEq a = CantInferEq (FooT a)
deriving instance (Eq (FooT a)) => Eq (CantInferEq a)
cantInferEq :: (Eq (FooT a)) => Proxy a -> FooT a -> FooT a -> Bool
cantInferEq _ x y = x == y
或者，您可以通过使用CantInferEq构造函数打包Eq (FooT a)字典来避免使用UndecidableInstances ：
{-# LANGUAGE GADTs, StandaloneDeriving #-}
data CantInferEq a where
CantInferEq :: (Eq (FooT a)) => FooT a -> CantInferEq a
deriving instance Eq (CantInferEq a)
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