使用Data.Constraint.Forall具有等式约束 [英] Using Data.Constraint.Forall with equality constraints
问题描述
{ - #LANGUAGE ScopedTypeVariables# - }
class C a where
foo :: forall fa b。 (C(f a),C(f b))=> f a - > fb
foo = _
现在,如果我想移动<$在$ foo $ c $类型中的类型类约束的右侧显示c $ c> a
和 b
c>(假设我想用 foo
来实现一个类型类方法,它需要在 a
中是多态的)和 b
),可以使用 Data.Constraint.Forall
:
{ - #LANGUAGE ScopedTypeVariables# - }
{ - #LANGUAGE ConstraintKinds,TypeOperators# - }
import Data.Constraint
import Data.Constraint.Forall
foo':: forall f。 (ForallF C f)=>一个b。 f a - > f b
foo'= helper
其中
helper :: forall a b。 f a - > f b
helper = case(instF :: ForallF C f: - C(f a))
Sub Dict - >
的情况(instF :: ForallF C f:-C(f b))Sub Dict - > foo
现在,我的问题是,假设我将函数更改为涉及类型相等的东西:
{ - #LANGUAGE TypeFamilies,ScopedTypeVariables# - }
type family F a :: * - > *
bar :: forall f g a b。 (F(f a)〜g a,F(f b)〜g b)=> f a - > fb
bar = _
有没有办法让上述技术适应这个? / p>
以下是我试过的:
{ - #LANGUAGE TypeFamilies,ScopedTypeVariables # - }
{ - #LANGUAGE ConstraintKinds,TypeOperators# - }
import Data.Constraint
import Data.Constraint.Forall
type F'Eq fgx = F (fx)〜gx
bar':: forall f g。 (Forall(F'Eq f g))=>一个b。 f a - > f b
bar'=助手
其中
helper :: forall a b。 f a - > f b
helper = case(inst :: Forall(F'Eq f g): - F'Eq f g a)of
Sub Dict - > case(inst :: Forall(F'Eq f g): - F'Eq f g b)of
Sub Dict - > bar
但是(不出所料)这个失败是因为不饱和类型的同义词:
类型同义词
'F'Eq'
应该有3个参数,但已被赋予2
在表达式类型签名中:
Forall(F'Eq fg): - F'Eq fga
在表达式中:
(inst :: Forall(F'Eq fg): - F'Eq fga)
您可以使用一个类:
{ - #LANGUAGE MultiParamTypeClasses,FlexibleInstances,FlexibleContexts# - }
class(F(fx)〜gx)=> F'Eq f g x
instance(F(f x)〜g x)=> F'Eq f g x
bar':: forall f g。 (Forall(F'Eq f g))=>一个b。 f a - > f b
bar'=助手
其中
helper :: forall a b。 f a - > f b
helper = case(inst :: Forall(F'Eq f g): - F'Eq f g a)of
Sub Dict - > case(inst :: Forall(F'Eq f g): - F'Eq f g b)of
Sub Dict - > bar
Let's say I have a function like this:
{-# LANGUAGE ScopedTypeVariables #-}
class C a where
foo :: forall f a b. (C (f a), C (f b)) => f a -> f b
foo = _
Now, if I would like to move the scope of a
and b
to the right of the typeclass constraint in the type of foo
(let's say, because I want to use foo
to implement a typeclass method that needs to be polymorphic in a
and b
), it can done with a bit of legwork using Data.Constraint.Forall
:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE ConstraintKinds, TypeOperators #-}
import Data.Constraint
import Data.Constraint.Forall
foo' :: forall f. (ForallF C f) => forall a b. f a -> f b
foo' = helper
where
helper :: forall a b. f a -> f b
helper = case (instF :: ForallF C f :- C (f a)) of
Sub Dict -> case (instF :: ForallF C f :- C (f b)) of
Sub Dict -> foo
Now, my question is, suppose I change my function to something involving type equalities:
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
type family F a :: * -> *
bar :: forall f g a b. (F (f a) ~ g a, F (f b) ~ g b) => f a -> f b
bar = _
Is there a way to adapt the above technique to this?
Here's what I tried:
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
{-# LANGUAGE ConstraintKinds, TypeOperators #-}
import Data.Constraint
import Data.Constraint.Forall
type F'Eq f g x = F (f x) ~ g x
bar' :: forall f g. (Forall (F'Eq f g)) => forall a b. f a -> f b
bar' = helper
where
helper :: forall a b. f a -> f b
helper = case (inst :: Forall (F'Eq f g) :- F'Eq f g a) of
Sub Dict -> case (inst :: Forall (F'Eq f g) :- F'Eq f g b) of
Sub Dict -> bar
But (unsurprisingly) this fails because of the unsaturated type synonym:
Type synonym
‘F'Eq’
should have 3 arguments, but has been given 2In an expression type signature:
Forall (F'Eq f g) :- F'Eq f g a
In the expression:
(inst :: Forall (F'Eq f g) :- F'Eq f g a)
You can use a class:
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-}
class (F (f x) ~ g x) => F'Eq f g x
instance (F (f x) ~ g x) => F'Eq f g x
bar' :: forall f g. (Forall (F'Eq f g)) => forall a b. f a -> f b
bar' = helper
where
helper :: forall a b. f a -> f b
helper = case (inst :: Forall (F'Eq f g) :- F'Eq f g a) of
Sub Dict -> case (inst :: Forall (F'Eq f g) :- F'Eq f g b) of
Sub Dict -> bar
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