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
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