[GHC] #5936: Support for data families in generics

[GHC]

#5936: Support for data families in generics
------------------------------+---------------------------------------------
 Reporter:  reinerp           |          Owner:                  
     Type:  feature request   |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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 Currently {{{DeriveGeneric}}} doesn't handle data families correctly, so
 that the following module

 {{{
 {-# LANGUAGE TypeFamilies, DeriveGeneric #-}
 module FamilyDerive where
 import GHC.Generics

 data family F a
 data instance F Int = FInt deriving Generic
 }}}

 fails to compile, with error

 {{{
 /tmp/FamilyDerive.hs:6:15:
     Couldn't match type `Rep (F Int)' with `M1 t0 t1 (M1 t2 t3 U1)'
     The type variables `t0', `t1', `t2', `t3' are ambiguous
     Possible fix: add a type signature that fixes these type variable(s)
     Expected type: Rep (F Int) x
       Actual type: M1 t0 t1 (M1 t2 t3 U1) x
     In the pattern: M1 (M1 U1)
     In an equation for `to': to (M1 (M1 U1)) = FInt
     In the instance declaration for `Generic (F Int)'
 }}}

 It turns out to require only small changes to GHC to handle data families.
 I've attached patches.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5936>
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