overlapping instances 7.10.1

[adam vogt]

Hi Sergei,

I think you should use {-# OVERLAPPABLE #-}: see the description here
https://ghc.haskell.org/trac/ghc/ticket/9242#comment:16 which is probably
in the manual somewhere too.

Regards,
Adam

On Thu, May 21, 2015 at 9:40 AM, Sergei Meshveliani <mechvel at botik.ru>
wrote:

> People,
>
> I wrote recently about finding places to set {-# OVERLAPPING #-}
> when porting an application from 7.8.2 to 7.10.1.
>
> I am doing this for porting  docon-2.12  from 7.8.2 to 7.10.1.
>
> And  ghc-7.10.1  indeed helped me to find several places to set this
> pragma (instead of using the total key -XOverlappingInstances).
>
> Finally, it has come to this module:
>
> ---------------------------------------------
> Preprocessing library docon-2.12.1...
> [48 of 86] Compiling ResRing__  ( ResRing__.hs, dist/build/ResRing__.o )
>
> ResRing__.hs:183:31:
>     Overlapping instances for Eq (Maybe PropValue)
>       arising from a use of ‘/=’
>     Matching instances:
>       instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
>       instance (Residue r, Eq a) => Eq (r a) -- Defined in ‘ResEuc0_’
>     In the expression: lookup IsGxBasis ps /= Just Yes
>     ...
> ----------------------------------------------
>
> As before, I set
>    instance {-# OVERLAPPING #-} (Residue r, Eq a) => Eq (r a)  where
>             ...
>
> in  ResEuc0_.hs.
>
> But this does not help, the compiler continues with the same report.
>
> I see the difference to previous porting process in that one of the
> overlapping instances is of the GHC library, so that I cannot set the
> needed overlap pragma for it.
>
> On the other hand, ghc-7.8.2 compiled docon-2.12 successfully.
>
> What my be a way out?
>
> Thanks,
>
> ------
> Sergei
>
>
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