我知道you can add constraints on associated type families and data families。这样做是对您班级的所有实例强制执行约束。
但我无法弄清楚如何在实例派生或函数声明中推断出这些约束。例如,此代码无法键入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?是否有解决方法来实现我想要的行为?
答案 0 :(得分:5)
问题的症结在于只给出一个FooT a
,你无处可以从中提取Eq
实例字典。
解决方法是在类型类要求中明确,从而有一个传递Eq
dict的地方:
{-# 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
或者您可以通过将UndecidableInstances
字典与Eq (FooT a)
构造函数打包来避免使用CantInferEq
:
{-# LANGUAGE GADTs, StandaloneDeriving #-}
data CantInferEq a where
CantInferEq :: (Eq (FooT a)) => FooT a -> CantInferEq a
deriving instance Eq (CantInferEq a)