"could not deduce ... from the context ..."，尽管上下文包含必要的内容

Question

以下文件编译失败：

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
module AltMonad.Monoid where
import AltMonad.Category
import Prelude (curry)
class Category c
   => Monoid i p c m where
  mid   :: i `c` m
  mcomb :: (m `p` m) `c` m
class Monoid () (,) (->) m
   => HaskellMonoid m where
  empty  :: m
  append :: m -> m -> m
instance Monoid () (,) (->) m
      => HaskellMonoid m where
  empty  = mid ()
  append = curry mcomb

它给出了错误：

[3 of 3] Compiling AltMonad.Monoid  ( AltMonadMonoid.hs, interpreted )
AltMonadMonoid.hs:24:12:
    Could not deduce (Monoid () p0 (->) m)
      arising from a use of `mid'
    from the context (Monoid () (,) (->) m)
      bound by the instance declaration
      at AltMonadMonoid.hs:(22,10)-(23,24)
    The type variable `p0' is ambiguous
    Relevant bindings include
      empty :: m (bound at AltMonadMonoid.hs:24:3)
    In the expression: mid ()
    In an equation for `empty': empty = mid ()
    In the instance declaration for `HaskellMonoid m'
AltMonadMonoid.hs:25:18:
    Could not deduce (Monoid i0 (,) (->) m)
      arising from a use of `mcomb'
    from the context (Monoid () (,) (->) m)
      bound by the instance declaration
      at AltMonadMonoid.hs:(22,10)-(23,24)
    The type variable `i0' is ambiguous
    Relevant bindings include
      append :: m -> m -> m (bound at AltMonadMonoid.hs:25:3)
    In the first argument of `curry', namely `mcomb'
    In the expression: curry mcomb
    In an equation for `append': append = curry mcomb
Failed, modules loaded: AltMonad.Category, AltMonad.Hask.

在我看来，我给出了正确的背景，但我不知道如何说服 GHC 这一点。

---

相关文件：

AltMonad.Category

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeSynonymInstances #-}
module AltMonad.Category where
import AltMonad.Hask
class Category cat where
  id  :: cat a a
  (.) :: b `cat` c -> a `cat` b -> a `cat` c
instance Category Hask where
  id    = x -> x
  g . f = x -> g (f x)

AltMonad.Hask

{-# LANGUAGE NoImplicitPrelude #-}
module AltMonad.Hask where
type Hask = (->)

Answer 1

发生

此错误是因为当您调用 mid () 时，它具有类型 Monoid () p0 (->) m => i -> m ，并且没有任何限制p0成为您选择的特定p，因为p0不会出现在类型签名中=>符号的右侧。 解决此问题的一种方法是通过FunctionalDependencies扩展：

-- ...
class Category c => Monoid i p c m | m -> p where
    mid   :: c i m
    mcomb :: c (p m m) m
-- ...
instance Monoid () (,) (->) m => HaskellMonoid m where
    empty  = mid ()
    append = curry mcomb

然而，这对append不起作用，因为变量i现在必须受到约束。 这只是意味着我们还需要让它取决于我们对m的选择：

class Category c => Monoid i p c m | m -> i, m -> p where
    -- ...

现在它将编译。 这是否是你想做的，我不知道，但我知道它会让你的代码编译。