[Haskell-cafe] aggressiveness of functional dependencies

Nicolas Frisby nicolas.frisby at gmail.com
Mon Nov 6 17:09:35 EST 2006

I have a question about functional dependencies, instance contexts,
and type inference. A specific example and question is in the attached

In brief the question is: to what degree does type inference use the
functional dependencies of an instance's class and context? I believe
I am wishing it were more aggressive than it is. Please note that I
have not enabled overlapping instances.

Any suggestions regarding how to get the inferred type of |rite_t1| to
be the one I anticipated would be much appreciated. Of course, I would
also appreciate explanations of why I shouldn't anticipate it!

The rest of this message is a copy of the attached code.


I'm using GHC 6.6, but I see the same inferred types with 6.4.1.

> {-# OPTIONS -fglasgow-exts #-}
> {-# OPTIONS -fallow-undecidable-instances #-} -- for the coverage condition
> module FunDepEx where

A plain ole' isomorphism class.

> class Iso a b | a -> b, b -> a where
>     rite :: a -> b
>     left :: b -> a

Isomorphism lifts through the sum bifunctor.

> bifmap_either f g = either (Left . f) (Right . g)
> instance ( Iso f f', Iso g g'
>          ) => Iso (Either f g) (Either f' g') where
>     rite = bifmap_either rite rite
>     left = bifmap_either left left

Some types to play around with.

> newtype MyChar = MyChar Char deriving (Show, Eq)
> instance Iso MyChar Char where
>     rite (MyChar c) = c
>     left c = MyChar c
> instance Iso Char MyChar where
>     rite c = MyChar c
>     left (MyChar c) = c

My type inference confusion follows; the unit arguments are just to
suppress the monomorphism restriction.

> t1 :: Either Char a
> t1 = Left 'c'
> rite_t1 () = rite t1

The inferred type for rite_t1 is
rite_t1 :: (Iso (Either Char a) (Either f' g')) => () -> Either f' g'

Why isn't the inferred type of rite_t1 the same as the ascribed type
of rite_t1'?

> rite_t1' :: Iso b b' => () -> Either MyChar b'
> rite_t1' () = rite t1
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