我不知道如何实现或者约束,但是如果我们只是将等式进行逻辑或操作,就像你的例子一样,我们可以使用类型族和提升布尔值来改进你的类型类方法并使其封闭。这仅适用于GHC 7.6及以上版本;最后,我会提到它在GHC 7.8中的更好之处以及如何将其移植回GHC 7.4。
思路是这样的:就像我们可以声明一个值级函数isBananaColor :: Color -> Bool,同样也可以声明一个类型级函数IsBananaColor :: Color -> Bool:
type family IsBananaColor (c :: Color) :: Bool
type instance IsBananaColor Green = True
type instance IsBananaColor Yellow = True
type instance IsBananaColor Black = True
type instance IsBananaColor White = False
type instance IsBananaColor Red = False
type instance IsBananaColor Blue = False
type instance IsBananaColor Tawny = False
type instance IsBananaColor Purple = False
如果我们喜欢,甚至可以添加。
type BananaColor c = IsBananaColor c ~ True
我们随后对每种水果颜色重复此操作,并将
Fruit
定义为您第二个示例中的内容:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE TypeFamilies #-}
data Color = White | Red | Blue | Yellow | Green | Tawny | Purple | Black
data Fruit (c :: Color) where
Banana :: BananaColor c => Fruit c
Apple :: AppleColor c => Fruit c
Grape :: GrapeColor c => Fruit c
Orange :: OrangeColor c => Fruit c
type family IsBananaColor (c :: Color) :: Bool
type instance IsBananaColor Green = True
type instance IsBananaColor Yellow = True
type instance IsBananaColor Black = True
type instance IsBananaColor White = False
type instance IsBananaColor Red = False
type instance IsBananaColor Blue = False
type instance IsBananaColor Tawny = False
type instance IsBananaColor Purple = False
type BananaColor c = IsBananaColor c ~ True
type family IsAppleColor (c :: Color) :: Bool
type instance IsAppleColor Red = True
type instance IsAppleColor Green = True
type instance IsAppleColor White = False
type instance IsAppleColor Blue = False
type instance IsAppleColor Yellow = False
type instance IsAppleColor Tawny = False
type instance IsAppleColor Purple = False
type instance IsAppleColor Black = False
type AppleColor c = IsAppleColor c ~ True
type family IsGrapeColor (c :: Color) :: Bool
type instance IsGrapeColor Red = True
type instance IsGrapeColor Green = True
type instance IsGrapeColor White = True
type instance IsGrapeColor Blue = False
type instance IsGrapeColor Yellow = False
type instance IsGrapeColor Tawny = False
type instance IsGrapeColor Purple = False
type instance IsGrapeColor Black = False
type GrapeColor c = IsGrapeColor c ~ True
type family IsOrangeColor (c :: Color) :: Bool
type instance IsOrangeColor Tawny = True
type instance IsOrangeColor White = False
type instance IsOrangeColor Red = False
type instance IsOrangeColor Blue = False
type instance IsOrangeColor Yellow = False
type instance IsOrangeColor Green = False
type instance IsOrangeColor Purple = False
type instance IsOrangeColor Black = False
type OrangeColor c = IsOrangeColor c ~ True
(如果你愿意,你可以放弃 -XConstraintKinds 和 type XYZColor c = IsXYZColor c ~ True 类型,并将 Fruit 的构造函数定义为 XYZ :: IsXYZColor c ~ True => Fruit c。) 这样做的好处是什么,坏处又是什么呢?优点是你可以按照自己的方式定义类型,这肯定是个胜利;由于颜色是封闭的,没有人可以添加更多的类型家族实例并破坏此类实例。
然而,这种方法也有缺点。你无法获得所需的推理,告诉你
[Apple, Grape, Banana]
自动属于
Fruit Green
; 更糟糕的是,
[Apple, Grape, Banana]
具有完全有效的类型
(AppleColor c, GrapeColor c, BananaColor c) => [Fruit c]
。是的,没有办法单态化它,但GHC无法弄清楚它。坦率地说,我无法想象任何解决方案能给您这些属性,尽管我随时准备惊喜。这个解决方案的另一个明显问题是它太长了——你需要为每个
IsXYZColor
类型族定义所有八个颜色情况!(对于这种形式的解决方案,使用全新的类型族也很恼人,但不可避免。)
我之前提到过,GHC 7.8将会使这个过程更加方便;它将通过消除需要为每个
IsXYZColor
类列出每一个情况的需求来实现这一点。如何做到的呢?Richard Eisenberg等人在GHC HEAD中引入了“封闭重叠有序类型族”,并且它将在7.8中可用。关于这个主题,有一篇
提交给POPL 2014的论文(还有一个
扩展版本),Richard还写了
一篇介绍性的博客文章(其中的语法似乎已经过时)。
这个想法是允许像普通函数一样声明类型族实例:所有等式必须在一个地方声明(移除开放世界假设),并按顺序尝试,这样就可以允许重叠。大致如下:
type family IsBananaColor (c :: Color) :: Bool
type instance IsBananaColor Green = True
type instance IsBananaColor Yellow = True
type instance IsBananaColor Black = True
type instance IsBananaColor c = False
这段代码存在歧义,因为IsBananaColor Green
同时匹配第一个和最后一个方程式;但在普通函数中,它可以正常工作。因此,新的语法如下:
type family IsBananaColor (c :: Color) :: Bool where
IsBananaColor Green = True
IsBananaColor Yellow = True
IsBananaColor Black = True
IsBananaColor c = False
那个
type family ... where { ... }
块定义了你想要定义的类型族; 它表明这个类型族是封闭的、有序的和重叠的,如上所述。因此,在 GHC 7.8 中,代码将变成以下形式(未经测试,因为我没有在我的计算机上安装它):
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
data Color = White | Red | Blue | Yellow | Green | Tawny | Purple | Black
data Fruit (c :: Color) where
Banana :: IsBananaColor c ~ True => Fruit c
Apple :: IsAppleColor c ~ True => Fruit c
Grape :: IsGrapeColor c ~ True => Fruit c
Orange :: IsOrangeColor c ~ True => Fruit c
type family IsBananaColor (c :: Color) :: Bool where
IsBananaColor Green = True
IsBananaColor Yellow = True
IsBananaColor Black = True
IsBananaColor c = False
type family IsAppleColor (c :: Color) :: Bool where
IsAppleColor Red = True
IsAppleColor Green = True
IsAppleColor c = False
type IsGrapeColor (c :: Color) :: Bool where
IsGrapeColor Red = True
IsGrapeColor Green = True
IsGrapeColor White = True
IsGrapeColor c = False
type family IsOrangeColor (c :: Color) :: Bool where
IsOrangeColor Tawny = True
IsOrangeColor c = False
太好了,我们可以轻松阅读而不会感到无聊!事实上,您会注意到我在此代码中切换到明确的IsXYZColor c〜True
版本;我这样做是因为使用这些更短的定义时,额外四个类型同义词的样板变得更加明显和令人讨厌!
然而,让我们朝相反的方向走,使这段代码更丑陋。为什么?因为我的机器上仍然运行着 GHC 7.4,不支持具有非
*
结果类型的类型族。那我们该怎么办呢?我们可以使用类型类和功能依赖来模拟它。思路是,我们不再使用
IsBananaColor :: Color -> Bool
,而是使用一个类型类
IsBananaColor :: Color -> Bool -> Constraint
,并添加从颜色到布尔值的功能依赖。然后,当且仅当在更好的版本中
IsBananaColor c ~ b
满足时,
IsBananaColor c b
才能被满足;由于
Color
是封闭的,并且我们从它建立了功能依赖,这仍然给我们相同的属性,只是更丑陋(虽然主要是概念上如此)。以下是完整的代码:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleContexts #-}
data Color = White | Red | Blue | Yellow | Green | Tawny | Purple | Black
data Fruit (c :: Color) where
Banana :: BananaColor c => Fruit c
Apple :: AppleColor c => Fruit c
Grape :: GrapeColor c => Fruit c
Orange :: OrangeColor c => Fruit c
class IsBananaColor (c :: Color) (b :: Bool) | c -> b
instance IsBananaColor Green True
instance IsBananaColor Yellow True
instance IsBananaColor Black True
instance IsBananaColor White False
instance IsBananaColor Red False
instance IsBananaColor Blue False
instance IsBananaColor Tawny False
instance IsBananaColor Purple False
type BananaColor c = IsBananaColor c True
class IsAppleColor (c :: Color) (b :: Bool) | c -> b
instance IsAppleColor Red True
instance IsAppleColor Green True
instance IsAppleColor White False
instance IsAppleColor Blue False
instance IsAppleColor Yellow False
instance IsAppleColor Tawny False
instance IsAppleColor Purple False
instance IsAppleColor Black False
type AppleColor c = IsAppleColor c True
class IsGrapeColor (c :: Color) (b :: Bool) | c -> b
instance IsGrapeColor Red True
instance IsGrapeColor Green True
instance IsGrapeColor White True
instance IsGrapeColor Blue False
instance IsGrapeColor Yellow False
instance IsGrapeColor Tawny False
instance IsGrapeColor Purple False
instance IsGrapeColor Black False
type GrapeColor c = IsGrapeColor c True
class IsOrangeColor (c :: Color) (b :: Bool) | c -> b
instance IsOrangeColor Tawny True
instance IsOrangeColor White False
instance IsOrangeColor Red False
instance IsOrangeColor Blue False
instance IsOrangeColor Yellow False
instance IsOrangeColor Green False
instance IsOrangeColor Purple False
instance IsOrangeColor Black False
type OrangeColor c = IsOrangeColor c True
-XDataKinds
,Green
是一个完全有效的类型级别的东西,其种类为Color
。这使得类型级别编程更加类型安全(因为你有编码的东西,Fruit Int
是良种的),并且消除了定义虚假data Green = Green
类型的需要。 - Antal Spector-Zabusky