首先,一些样板:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances, UndecidableSuperClasses #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}
module ConstrainApplications where
import GHC.Exts (Constraint)
import Data.Type.Equality
现在输入族来解构任意类型的应用程序。
type family GetFun a where
GetFun (f _) = f
type family GetArg a where
GetArg (_ a) = a
现在是一个非常通用的类型函数,比回答问题所需的更通用。但这允许涉及应用程序的两个组件的约束。
type G (cfa :: (j -> k) -> j -> Constraint) (fa :: k)
= ( fa ~ (GetFun fa :: j -> k) (GetArg fa :: j)
, cfa (GetFun fa) (GetArg fa))
我不喜欢提供没有类匹配的约束函数,所以这是一个一流的版本G
.
class G cfa fa => GC cfa fa
instance G cfa fa => GC cfa fa
可以表达F
using G
和一个辅助类:
class (cf f, ca a) => Q cf ca f a
instance (cf f, ca a) => Q cf ca f a
type F cf ca fa = G (Q cf ca) fa
class F cf ca fa => FC cf ca fa
instance F cf ca fa => FC cf ca fa
以下是一些示例用途F
:
t1 :: FC ((~) Maybe) Eq a => a -> a -> Bool
t1 = (==)
-- In this case, we deconstruct the type *twice*:
-- we separate `a` into `e y`, and then separate
-- `e` into `Either x`.
t2 :: FC (FC ((~) Either) Show) Show a => a -> String
t2 x = case x of Left p -> show p
Right p -> show p
t3 :: FC Applicative Eq a => a -> a -> GetFun a Bool
t3 x y = (==) <$> x <*> y