[Haskell-cafe] Tagged instances for Aeson overlapped

Adam Bergmark adam at bergmark.nl
Mon Mar 7 15:24:22 UTC 2016

Whoops, didn’t reply-all!

One instance is in Data.Aeson.Types.Instances, where are you getting the other one from?

Adam Bergmark

On Mon 07 Mar 2016 at 15:37 Dmitry Olshansky

mailto:Dmitry Olshansky <olshanskydr at gmail.com>
> wrote:


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 =>


                     (Data.Tagged.Tagged a b)

            -- Defined in ‘aeson-’

          instance forall k x (r :: k).

                   aeson- x =>


                     (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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