为什么 GHC 不会减少我的类型家族？

Question

这是一个无类型的 lambda 演算，其项由其自由变量索引。我正在使用 singletons 库来获取类型级字符串的单例值。

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}

import Data.Singletons
import Data.Singletons.TypeLits

data Expr (free :: [Symbol]) where
    Var :: Sing a -> Expr '[a]
    Lam :: Sing a -> Expr as -> Expr (Remove a as)
    App :: Expr free1 -> Expr free2 -> Expr (Union free1 free2)

A Var引入了一个自由变量。 lambda 抽象绑定了一个在主体中显示为自由的变量（如果有一个匹配）。应用程序连接表达式两部分的自由变量，删除重复项（因此 x y 的自由变量是 x 和 y，而 x x 的自由变量只是 x）。我写出了那些类型族：

type family Remove x xs where
    Remove x '[] = '[]
    Remove x (x ': xs) = Remove x xs
    Remove x (y ': xs) = y ': Remove x xs

type family Union xs ys where
    Union xs ys = Nub (xs :++ ys)

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

type family Nub xs where
    Nub xs = Nub' '[] xs

type family Nub' seen xs where
    Nub' seen '[] = '[]
    Nub' seen (x ': xs) = If (Elem x seen) (Nub' seen xs) (Nub' (x ': seen) (x ': xs))

type family If c t f where
    If True t f = t
    If False t f = f

type family Elem x xs where
    Elem x '[] = False
    Elem x (x ': xs) = True
    Elem x (y ': xs) = Elem x xs

我在交互式提示下测试了这个：

ghci> :t Var (sing :: Sing "x")
Var (sing :: Sing "x") :: Expr '["x"]  -- good
ghci> :t (Lam (sing :: Sing "x") (Var (sing :: Sing "x")))
(Lam (sing :: Sing "x") (Var (sing :: Sing "x")))
  :: Expr (Remove "x" '["x"])  -- not so good

我很惊讶地发现恒等函数 \x. x 的类型是 Expr (Remove "x" '["x"])，而不是 Expr '[]。 GHC 似乎不愿意评估类型族 Remove。我做了更多实验，了解到这不是我的类型家族本身的问题 - GHC 很乐意在这种情况下减少它：

ghci> :t (Proxy :: Proxy (Remove "x" '["x"]))
(Proxy :: Proxy (Remove "x" '["x"])) :: Proxy '[]

所以：当我查询我的 GADT 类型时，为什么 GHC 不会将 Remove "x" '["x"] 减少到 '[]？一般来说，类型检查器何时会或不会评估类型族？我可以使用启发式方法来避免对其行为感到惊讶吗？

Answer 1

有效。 GHC 似乎只是懒惰。

λ *Main > :t (Lam (Proxy :: Proxy "x") (Var (Proxy :: Proxy "x")))
(Lam (Proxy :: Proxy "x") (Var (Proxy :: Proxy "x")))
  :: Expr (Remove "x" '["x"])

λ *Main > :t (Lam (Proxy :: Proxy "x") (Var (Proxy :: Proxy "x"))) :: Expr '[]
(Lam (Proxy :: Proxy "x") (Var (Proxy :: Proxy "x"))) :: Expr '[]
  :: Expr '[]

λ *Main > :t (Lam (Proxy :: Proxy "x") (Var (Proxy :: Proxy "x"))) :: Expr '["x"]

<interactive>:1:2:
    Couldn't match type ‘'[]’ with ‘'["x"]’
    Expected type: Expr '["x"]
      Actual type: Expr (Remove "x" '["x"])
    In the expression:
        (Lam (Proxy :: Proxy "x") (Var (Proxy :: Proxy "x"))) ::
          Expr '["x"]

我更改了定义，因此不依赖单例库（更易于临时测试）：

{-# LANGUAGE TypeOperators, DataKinds, TypeFamilies, GADTs #-}

import Data.Proxy
import GHC.TypeLits

type family Remove (x :: Symbol) (xs :: [Symbol]) where
    Remove x '[] = '[]
    Remove x (x ': xs) = Remove x xs
    Remove x (y ': xs) = y ': Remove x xs

data Expr (free :: [Symbol]) where
    Var :: KnownSymbol a => Proxy a -> Expr '[a]
    Lam :: KnownSymbol a => Proxy a -> Expr as -> Expr (Remove a as)
--    App :: Expr free1 -> Expr free2 -> Expr (Union free1 free2)

为什么 GHC 不会减少我的类型家族？

Why won't GHC reduce my type family?

haskell

lambda-calculus

type-families

gadt