Re: [GHC] #12438: DeriveDataTypeable - deriving instance Data (Mu (Const ()))

26 Jul 2016


      #12438: DeriveDataTypeable - deriving instance Data (Mu (Const ()))
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        Reporter:  lspitzner         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by lspitzner:

@@ -2,1 +2,1 @@
- {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving, PolyKinds #-}
+ {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}

New description:

 {{{#!hs
 {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
 import qualified Data.Data

 data Mu f = Mu (f (Mu f))
 deriving instance Data.Data.Data (Mu (Const ()))
 }}}

 produces

 {{{
     • No instance for (Data (Const () (Mu (Const ()))))
         arising from a use of ‘k’
     • In the expression: (z Mu `k` a1)
       In an equation for ‘gfoldl’: gfoldl k z (Mu a1) = (z Mu `k` a1)
       When typechecking the code for ‘gfoldl’
         in a derived instance for ‘Data (Mu (Const ()))’:
         To see the code I am typechecking, use -ddump-deriv
       In the instance declaration for ‘Data (Mu (Const ()))’
 }}}

 while other type constructors work, e.g.

 {{{#!hs
 deriving instance Data.Data.Data (Mu [])
 deriving instance Data.Data.Data (Mu ((,) ()))
 }}}

 i am not sure if #10835 is related.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12438#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler

    