我正在尝试将列表的单例版本简化。我很难解构它。这是一个最小的实现:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ExplicitForAll #-}
module InductiveWrapper where
import Data.Kind (Type)
import Data.Proxy (Proxy)
import GHC.Prim (coerce)
data List a = Nil | Cons a (List a)
data SList :: [ k ] -> Type where
SNil :: SList '[]
SCons :: Proxy k -> SList ks -> SList (k ': ks)
newtype Set a = S [ a ]
data SSet :: Set k -> Type where
SS :: SList xs -> SSet ('S xs)
type family Add (el :: k) (set :: Set k) :: Set k where
Add el ('S xs) = 'S (el ': xs)
uncons :: forall k (el :: k) (set :: Set k)
. SSet (Add el set) -> (Proxy el, SSet set)
uncons (SS (x `SCons` xs)) = (x, SS xs)
以下是错误的相关位:
Could not deduce: set ~ 'S ks
from the context: Add el set ~ 'S xs
bound by a pattern with constructor:
SS :: forall k (xs :: [k]). SList xs -> SSet ('S xs),
in an equation for ‘uncons’
[...]
or from: xs ~ (k1 : ks)
bound by a pattern with constructor:
SCons :: forall k1 (k2 :: k1) (ks :: [k1]).
Proxy k2 -> SList ks -> SList (k2 : ks),
[...]
• Relevant bindings include
xs :: SList ks (bound at InductiveWrapper.hs:37:29)
x :: Proxy k1 (bound at InductiveWrapper.hs:37:19)
xs' :: SList xs (bound at InductiveWrapper.hs:37:14)
s :: SSet (Add el set) (bound at InductiveWrapper.hs:37:8)
据我所知,问题是Add el set
被卡住了,因为类型检查器不了解可以构造set
的唯一方法是使用'S
。
我如何解开它或通过其他方式解决此问题?除了使用type
代替newtype
。我这样做的全部原因是完全隐藏了[ k ]
和SList
的使用。
类型族是非单射的,从技术上讲,这意味着您不能从结果到参数,从右到左。除了没有GHC 8.0引入了TypeFamilyDependencies
,可让您指定类型族的插入性,如下所示:
type family Add (el :: k) (set :: Set k) = (set' :: Set k) | set' -> el set where
Add el ('S xs) = 'S (el ': xs)
但是,由于某些原因,我尚不完全了解,这在您的情况下仍然不起作用,导致了同样的问题。我怀疑这可能与问题清单是双重包装的事实有关,不确定。
但是我确实有另一个解决方法:您可以放弃整个内射性问题,并以其他方式指定类型族-从列表到元组。除非您需要two类型族-一个用于头部,一个用于尾巴:
type family Head set where Head ('S (el ': xs)) = el
type family Tail set where Tail ('S (el ': xs)) = 'S xs
uncons :: SSet set -> (Proxy (Head set), SSet (Tail set))
uncons (SS (x `SCons` xs)) = (x, SS xs)
但是这对我来说似乎有点过分设计。如果您只需要取消约束这些类型集,那么我会推荐一个好的ol'type类,它具有将类型和值包装在一起的无与伦比的优势,因此您不必为了手动匹配它们而跳了圈:
class Uncons set res | set -> res where
uncons :: SSet set -> res
instance Uncons ('S (el ': xs)) (Proxy el, SSet ('S xs)) where
uncons (SS (x `SCons` xs)) = (x, SS xs)