[Haskell-cafe] Tagged instances for Aeson overlapped

Dmitry Olshansky olshanskydr at gmail.com
Mon Mar 7 14:37:37 UTC 2016


I tried to compile my project with GHC 8 (from HEAD).

And I got

>     • Overlapping instances for
> aeson-
> ...
>         arising from a use of ‘serve’
>       Matching instances:
>         two instances involving out-of-scope types
>           instance aeson- b =>
>                    aeson-
>                      (Data.Tagged.Tagged a b)
>             -- Defined in ‘aeson-’
>           instance forall k x (r :: k).
>                    aeson- x =>
>                    aeson-
>                      (Data.Tagged.Tagged r x)

One of instances should be removed probably.
I suppose that instance for removing should be instance in Data.Tagged.
But version in Aeson should be PolyKinded.

What do you think?

Best regards,
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