This has been reported: https://ghc.haskell.org/trac/ghc/ticket/8338
But it's really not clear what the solution is! Richard On Aug 11, 2014, at 9:27 PM, Iavor Diatchki <iavor.diatc...@gmail.com> wrote: > Hello, > > this is clearly a bug in GHC: where `B` and `C` are imported, there should > have been an error, saying that there is a duplicate instance of `Foo Int`. > If there is no ticket for this already, could you please add one? > > -Iavor > > > > > > > On Mon, Aug 11, 2014 at 12:35 PM, Dan Doel <dan.d...@gmail.com> wrote: > On Mon, Aug 11, 2014 at 11:36 AM, Twan van Laarhoven <twa...@gmail.com> wrote: > To me, perhaps naively, IncoherentInstances is way more scary than > OverlappingInstances. > > It might be a bit naive. Most things that incoherent instances would allow > are allowed with overlapping instances so long as you partition your code > into two modules. So unless such a partitioning is impossible, overlapping > instances are almost as scary as incoherent instances (unless the module > separation somehow makes it less scary). > > And actually, with the way GHC handles instances, you can get more incoherent > behavior than incoherent instances allow without enabling any extensions, > just using modules: > > module A where > class Foo a where foo :: a > > module B where > import A > instance Foo Int where foo = 5 > bar :: Int ; bar = foo > > module C where > import A > instance Foo Int where foo = 6 > baz :: Int ; baz = foo > > module D where > import B > import C > > quux = bar + baz -- 11 > > -- Dan > > _______________________________________________ > Libraries mailing list > librar...@haskell.org > http://www.haskell.org/mailman/listinfo/libraries > > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
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