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我正在尝试使用类型 [*] 的幻像类型折叠数据。这是我的代码的简化版本

{-# LANGUAGE DataKinds, KindSignatures #-}

module Stack where
import Data.HList
import Data.Foldable as F

data T (a :: [*]) = T (Tagged a String)
(!++!) :: T a -> T b -> T (HAppendList a b)
(T a) !++! (T b) = T (Tagged (untag a ++ untag b))


a = T (Tagged "1") :: T '[Int]
b = T (Tagged "-- ") :: T '[]
ab = a !++! b :: T '[Int]

我想要一个折叠运算符

(!++*) :: (Foldable t ) =>  T a -> t (T '[]) -> T a
a !++* t = F.foldl (!++!) a t

但这不起作用。a和不同的编译器HAppendList a '[]，即使它们不是。

为什么编译不能统一HAppendList a '[]和a？

（虽然我不能在 ghci 中手动折叠:t a !++! b !++! b !++! b => T '[Int]

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1 回答 1

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注意 的定义HAppendList：

type family HAppendList (l1 :: [k]) (l2 :: [k]) :: [k]
type instance HAppendList '[] l = l
type instance HAppendList (e ': l) l' = e ': HAppendList l l'

你我都知道这[]是 的左右标识++，但编译器只知道左边标识：

happend' :: T a -> T b -> T (HAppendList a b)
happend' (T (Tagged a)) (T (Tagged b)) = (T (Tagged (a++b)))

-- Doesn't typecheck 
leftIdentity' :: T a -> T '[] -> T a 
leftIdentity' x y = happend' x y 

rightIdentity' :: T '[] -> T a -> T a 
rightIdentity' x y = happend' x y

你需要有

type instance HAppendList '[] l = l
type instance HAppendList l '[] = l
type instance HAppendList (e ': l) l' = e ': HAppendList l l'

让编译器知道左右身份；但这些会重叠，所以它不会进行类型检查。但是，您可以翻转争论：

(!+++!) :: T a -> T b -> T (HAppendList a b) 
(!+++!) (T (Tagged x)) (T (Tagged y)) = T (Tagged (y ++ x))

(!++*) :: Foldable t => T a -> t (T '[]) -> T a
a !++* t = F.foldl (flip (!+++!)) a t

使用 ghc 7.8 中引入的封闭类型族，您可以解决此问题：

type family (++) (a :: [k]) (b :: [k]) :: [k] where 
  '[] ++ x = x
  x ++ '[] = x
  (x ': xs) ++ ys = x ': (xs ++ ys)

happend :: T a -> T b -> T (a ++ b)
happend (T (Tagged a)) (T (Tagged b)) = (T (Tagged (a++b)))

leftIdentity :: T a -> T '[] -> T a 
leftIdentity x y = happend x y 

rightIdentity :: T '[] -> T a -> T a 
rightIdentity x y = happend x y
于 2014-06-16T23:30:37.570 回答