Re: [GHC] #10524: PolyKinds doesn't interact well with DeriveFunctor

25 Jun 2015

      #10524: PolyKinds doesn't interact well with DeriveFunctor
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #10561            |  Differential Revisions:
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Comment (by RyanGlScott):

 It looks like {{{PolyKinds}}} also breaks deriving {{{Data}}} instances as
 well:

 {{{#!hs
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE PolyKinds #-}

 import Data.Data

 newtype WrappedFunctor f a = WrapFunctor (f a) deriving (Data, Typeable)
 }}}

 This will result in the error:

 {{{
     No instance for (Typeable a)
       arising from the 'deriving' clause of a data type declaration
     Possible fix:
       use a standalone 'deriving instance' declaration,
         so you can specify the instance context yourself
     When deriving the instance for (Data (WrappedFunctor f a))
 }}}

 (Presumably, it's trying to infer the instance context {{{Data (f a),
 Typeable f, Typeable a) => Data (WrappedFunctor f a)}}}, but can't.)

 I'm not sure if this is due to the same underlying issue, but it seems
 likely, since that code will compile without {{{PolyKinds}}}. I reproduced
 this on GHC 7.8, 7.10, and HEAD.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:7
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