Hi, AntC's f can be done without -XOverlappingInstances < http://lpaste.net/7559485273839501312>, using the trick didn't work in #9918. I'm not sure extra syntax is justified to clean up this rare case.
Regards, Adam On Sun, Jun 7, 2015 at 11:12 AM, Dan Doel <dan.d...@gmail.com> wrote: > It seems to me the problem is that there's no way to define classes by > consecutive cases to match the family definitions. I don't know what a good > syntax for that would be, since 'where' syntax is taken for those. But it > seems like it would correspond fill the hole here. > > On Sun, Jun 7, 2015 at 7:27 AM, Richard Eisenberg <e...@cis.upenn.edu> > wrote: > >> 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 >> > > > _______________________________________________ > 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
