[Haskell-cafe] Ambiguous reified dictionaries

[Martijn van Steenbergen]

Good morning,

The [1]GHC user's guide, section 8.4.5 says:

"The new feature is that pattern-matching on MkSet (as in the definition 
of insert) makes available an (Eq a) context. In implementation terms, 
the MkSet constructor has a hidden field that stores the (Eq a) 
dictionary that is passed to MkSet; so when pattern-matching that 
dictionary becomes available for the right-hand side of the match."

But what happens if there are several dictionaries available?

Consider these three modules:

ReifyMonoid.hs:

> {-# LANGUAGE GADTs #-}
> 
> module ReifyMonoid where
> 
> import Data.Monoid
> 
> data MonoidInst a where
>   MkMonoidInst :: Monoid a => MonoidInst a

ReifySum.hs:

> module ReifySum where
> 
> import ReifyMonoid
> import Data.Monoid
> 
> instance Monoid Int where
>   mempty = 0
>   mappend = (+)
> 
> intSum :: MonoidInst Int
> intSum = MkMonoidInst

ReifyProd.hs:

> module ReifyProd where
> 
> import ReifyMonoid
> import Data.Monoid
> 
> instance Monoid Int where
>   mempty = 1
>   mappend = (*)
> 
> intProd :: MonoidInst Int
> intProd = MkMonoidInst

Now a function

> emp :: MonoidInst a -> a
> emp MkMonoidInst = mempty

works as you'd expect:

*ReifySum ReifyProd> emp intSum
0
*ReifySum ReifyProd> emp intProd
1

But what about this function?

> empAmb :: MonoidInst a -> MonoidInst a -> a
> empAmb MkMonoidInst MkMonoidInst = mempty

Now there are two dictionaries available. GHC consistently picks the one 
from the second argument:

*ReifySum ReifyProd> empAmb intProd intSum
1
*ReifySum ReifyProd> empAmb intSum intProd
0

My questions are:

1) Shouldn't GHC reject this as being ambiguous?
2) Should class constraints only be available on existentially qualified 
type variables to prevent this from happening at all?
3) Is it possible to implement the following function?

> mkMonoidInst :: a -> (a -> a -> a) -> MonoidInst a
> mkMonoidInst mempty mappend = ...

Thank you,

Martijn.



[1] 
http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#gadt-style