[GHC] #12245: Deriving Data at higher kinds

29 Jun 2016


      #12245: Deriving Data at higher kinds
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Here's what
 [https://mail.haskell.org/pipermail/generics/2016-June/000564.html Lennart
 Spitzner wanted to do]:
 {{{
...
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE FlexibleInstances #-}
import Data.Data ( Data )
data Foo f = Foo (f Bool) (f Int)
deriving instance Data (Foo [])
deriving instance Data (Foo Maybe)
 }}}
 Of course you can't derive `Data` for `Foo` because we don't know what `f`
 is, so Lennart is making multiple instances, one for each instance of `f`.
 It's a bit clumsy. What we would really like is
 {{{
 deriving instance (forall a. Data  => Data (f a)) => Data (Foo f)
 }}}
 but we don't have higher order instances yet!  So Lennart is manually
 making two instances.
This should work, but he gets
 {{{
...
Main.hs: line 45, column 1:
  Multiple declarations of ‘$cr2C’
    Declared at: Main.hs:44:1
                 Main.hs:45:1
Main.hs: line 45, column 1:
  Multiple declarations of ‘$tr2D’
    Declared at: Main.hs:44:1
                 Main.hs:45:1
 }}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12245
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