尽管上下文认为有必要，但“无法根据上下文推论……”

Question

The following file fails to compile:

{-# 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

It gives the errors:

[3 of 3] Compiling AltMonad.Monoid  ( AltMonad\Monoid.hs, interpreted )

AltMonad\Monoid.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 AltMonad\Monoid.hs:(22,10)-(23,24)
    The type variable `p0' is ambiguous
    Relevant bindings include
      empty :: m (bound at AltMonad\Monoid.hs:24:3)
    In the expression: mid ()
    In an equation for `empty': empty = mid ()
    In the instance declaration for `HaskellMonoid m'

AltMonad\Monoid.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 AltMonad\Monoid.hs:(22,10)-(23,24)
    The type variable `i0' is ambiguous
    Relevant bindings include
      append :: m -> m -> m (bound at AltMonad\Monoid.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.

It seems to me that I'm giving the right context, but I don't know how to convince GHC of this.

---

Related files:

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

This error occurs because when you call mid () , it has the type Monoid () p0 (->) m => i -> m , and there is nothing constraining p0 to be the particular p you chose since p0 does not appear on the right hand side of the => sign in the type signature. One way to work around this is through the FunctionalDependencies extension:

-- ...

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

However, this won't work for append , as the variable i now has to be constrained. This just means that we need to have it dependent on our choice for m as well:

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

And now it'll compile. Whether or not this is what you want to do, I don't know, but I do know that it'll get your code compiling.