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 F > oo 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 > >
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