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.doel@gmail.com> wrote:

On Mon, Aug 11, 2014 at 11:36 AM, Twan van Laarhoven <twanvl@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​

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