This is all expected behavior. GHC's lazy overlap checking for class instances simply cannot apply to type families -- it would be unsound. I'm afraid I don't see what can be improved here.
Richard On Jun 6, 2015, at 2:04 AM, AntC <anthony_clay...@clear.net.nz> wrote: >> From: AntC >> Date: 2015-06-04 22:39:25 GMT >> >> Take the standard example for partial overlaps. >> Suppose I have a class: ... > >> I'm also getting (in more complex examples) >> GHC complaining it can't infer the types >> for the result of f. >> So now I'm having to put type equality >> constraints on the class instances, >> to assure it that F comes up with >> the right type. > > In a reduced example, I'm still getting > poor type checking. This is GHC 7.8.3. > This seems so dumb, I'm suspecting a defect, > It's similar to > but much more glaring than: > https://ghc.haskell.org/trac/ghc/ticket/10227 > https://ghc.haskell.org/trac/ghc/ticket/9918 > > {-# LANGUAGE TypeFamilies, > FlexibleInstances > #-} > module ClosedTypeFamily where > > data Foo b c = Foo b c deriving (Eq, Read, Show) > > type family F a where > F (Foo Int c) = Int -- Foo Int is first instance > F (Foo b Char) = Char > > class C a where f :: a -> F a > > instance C (Foo Int c) where -- compiles OK > f (Foo x _) = x > > instance (F (Foo b Char) ~ Char) => C (Foo b Char) where > f (Foo _ y) = y > > needs the eq constraint. Without it, GHC complains: > Couldn't match expected type ‘F (Foo b Char)’ > with actual type ‘Char’ > Relevant bindings include > f :: Foo b Char -> F (Foo b Char) > In the expression: y > In an equation for ‘f’: f (Foo _ y) = y > > Note that if I change the sequence > of the family instances for F, > then GHC instead complains > about the class instance for (Foo Int c). > > OK these are overlapping class instances. > But GHC's usual behaviour > (without closed type families) > is to postpone complaining > until and unless a usage > (Foo Int Char) actually turns up. > > BTW if I put a first family instance > F (Foo Int Char) = Int > to explicitly catch the overlap, > then GHC complains about **both** class instances. > > Reminder [to Richard] > I need not only types but also terms. > > AntC > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
