上下文中的类型变量不固定？答案

【问题标题】：Type variables in context not fixed?上下文中的类型变量不固定？
【发布时间】：2018-09-28 21:53:25
【问题描述】：

我目前正在试验类型级代码。我有一个实例，其类型变量只出现在上下文中，而不出现在实例本身中。不知何故，编译器不喜欢那样，但我不知道为什么不。向HasRecipe effect pot target -> deps 添加函数依赖项可以解决该错误，但随后在代码最后一行的测试中推断出不正确的deps。

错误：

• Could not deduce (HasRecipe target pot effect deps0)
  from the context: (HasRecipe target pot effect deps,
                     SubSelect pot deps)
    bound by an instance declaration:
               forall target (pot :: [*]) (effect :: * -> *) (deps :: [*]).
               (HasRecipe target pot effect deps, SubSelect pot deps) =>
               CanCook target pot effect
    at Lib.hs:15:10-92
  The type variable ‘deps0’ is ambiguous
• In the ambiguity check for an instance declaration
  To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
  In the instance declaration for ‘CanCook target pot effect’

来源：

#!/usr/bin/env stack
-- stack --resolver lts-11.4 --install-ghc runghc
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE PolyKinds #-}

import Data.Proxy

class HEq (x :: k) (y :: k) (b :: Bool) | x y -> b

data family HList (l::[*])

data instance HList '[] = HNil
data instance HList (x ': xs) = x `HCons` HList xs

deriving instance Eq (HList '[])
deriving instance (Eq x, Eq (HList xs)) => Eq (HList (x ': xs))

deriving instance Ord (HList '[])
deriving instance (Ord x, Ord (HList xs)) => Ord (HList (x ': xs))

deriving instance Bounded (HList '[])
deriving instance (Bounded x, Bounded (HList xs)) => Bounded (HList (x ': xs))

class HExtend e l where
  type HExtendR e l
  (.*.) :: e -> l -> HExtendR e l

infixr 2 .*.

instance HExtend e (HList l) where
  type HExtendR e (HList l) = HList (e ': l)
  (.*.) = HCons


main = pure ()

newtype Recipe effect target (deps :: [*]) = Recipe { runRecipe :: HList deps -> effect target }

class CanCook target (pot :: [*]) effect | pot -> effect where
  cook :: HList pot -> effect target

instance (HasRecipe target pot effect deps, SubSelect pot deps) => CanCook target pot effect where
  cook pot =
    let
      deps :: HList deps
      deps = subselect pot
      r :: Recipe effect target deps
      r = recipe pot
    in
      runRecipe r $ deps

type family PotEffect (pot :: [*]) where
  PotEffect (Recipe effect _ _ ': '[]) = effect
  PotEffect (Recipe effect _ _ ': tail) = effect

class HasRecipe target (pot :: [*]) effect deps | pot -> effect where
  recipe :: HList pot -> Recipe effect target deps

class SubSelect (pot :: [*]) (deps :: [*]) where
  subselect :: HList pot -> HList deps

instance SubSelect t f where
  subselect = undefined

class HasRecipeCase (b :: Bool) (target :: *) (pot :: [*]) effect (deps :: [*]) | pot -> effect where
  recipeCase :: Proxy b -> Proxy target -> HList pot -> Recipe effect target deps

instance HasRecipeCase True target ((Recipe effect target deps) ': leftoverPot) effect deps where
  recipeCase _ _ (HCons head _) = head

instance (HasRecipe target leftoverPot effect deps) =>
  HasRecipeCase False target ((Recipe effect target1 deps) ': leftoverPot) effect deps where
  recipeCase _ _ (HCons _ tail) = recipe tail

instance (HEq target t bool, HasRecipeCase bool target pot effect deps, pot ~ ((Recipe effect t deps) ': leftoverPot)) =>
  HasRecipe target ((Recipe effect t deps) ': leftoverPot) effect deps where
  recipe = undefined

newtype M1 = M1 ()
newtype M2 = M2 ()
newtype M3 = M3 ()
newtype M4 = M4 ()

r1 :: Recipe IO M1 '[M2, M3]
r1 = undefined

r2 :: Recipe IO M2 '[]
r2 = undefined

r3 :: Recipe IO M3 '[M4]
r3 = undefined

r4 :: Recipe IO M4 '[]
r4 = undefined

cookbook1 = r1 .*. r2 .*. r3 .*. r4 .*. HNil

cookbook2 = r3 .*. r4 .*. r2 .*. r1 .*. HNil

c1 = cook cookbook1 :: IO M4
-- c2 = cook cookbook2 :: IO M4

【问题讨论】：

如果启用模糊类型会怎样？
同样的错误，但在调用方站点。
不应该SubSelect有一些实例吗？否则它不会限制任何东西。
我什至无法运行这个示例：lts-11.4 中没有包 hlist。您使用的是什么依赖项？例如，multistate 具有 Data.HList.HList 并位于 lts-11.4 中。你在用那个吗？
@chi 抱歉，在这里工作是因为我将其作为一个项目完成并将其导出为堆栈脚本 :-/ 。应该是HList-0.5.0.0

标签： haskell type-level-computation hlist

【解决方案1】：

你不能有这样的实例

instance (HasRecipe target pot effect deps, SubSelect pot deps)
       => CanCook target pot effect where

tyvar deps 出现在上下文中，但没有出现在头部，使实例不明确。

具体来说，可能有四个实例

instance HasRecipe target pot effect A
instance SubSelect pot A
instance HasRecipe target pot effect B
instance SubSelect pot B

而 GHC 无法选择你想要的那个。您必须在实例定义中选择deps，或者在cook 调用站点（启用模糊类型）。

如果有 target pot effect -> deps 这样的基金，这将消除歧义，但我很难理解其意图。

【讨论】：