Overlapping and incoherent instances

[Richard Eisenberg]

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.diatchki at 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.doel at gmail.com> wrote:
> On Mon, Aug 11, 2014 at 11:36 AM, Twan van Laarhoven <twanvl at 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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