[GHC] #10775: Enable PolyKinds in GHC.Generics

15 Aug 2015

      #10775: Enable PolyKinds in GHC.Generics
-------------------------------------+-------------------------------------
              Reporter:              |             Owner:
  RyanGlScott                        |
                  Type:  feature     |            Status:  new
  request                            |
              Priority:  normal      |         Milestone:
             Component:              |           Version:  7.10.2
  libraries/base                     |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 As suggested in
 [https://mail.haskell.org/pipermail/libraries/2015-July/025981.html this]
 Haskell libraries mailing list thread. Among other benefits, this would
 allow use of generic functions with `Proxy t` when `t` is of a kind other
 than `*`.

 There seem to be more changes required than just putting `{-# LANGUAGE
 PolyKinds #-}` in `GHC.Generics`, however, since I tried doing that myself
 and found myself unable to properly derive `Generic(1)` instances in
 `GHC.Generics`. Here is a snippet of the resulting error message:

 {{{
 libraries/base/GHC/Generics.hs:826:1: error:
     Couldn't match type ‘M1 i0 c0 (M1 i1 c1 U1) p0’ with ‘M1 D x’
     Expected type: Rep (Proxy t) x
       Actual type: M1 i0 c0 (M1 i1 c1 U1) p0
     Relevant bindings include
       from :: Proxy t -> Rep (Proxy t) x
         (bound at libraries/base/GHC/Generics.hs:826:1)
     In the expression: M1 (M1 U1)
     In an equation for ‘from’: from Proxy = M1 (M1 U1)
     When typechecking the code for ‘from’
       in a derived instance for ‘Generic (Proxy t)’:
       To see the code I am typechecking, use -ddump-deriv

 libraries/base/GHC/Generics.hs:826:1: error:
     Couldn't match type ‘M1 t0 t1 (M1 t3 t4 U1) t2’ with ‘M1 D x’
     Expected type: Rep (Proxy t) x
       Actual type: M1 t0 t1 (M1 t3 t4 U1) t2
     Relevant bindings include
       to :: Rep (Proxy t) x -> Proxy t
         (bound at libraries/base/GHC/Generics.hs:826:1)
     In the pattern: M1 (M1 U1)
     In an equation for ‘to’: to (M1 (M1 U1)) = Proxy
     When typechecking the code for ‘to’
       in a derived instance for ‘Generic (Proxy t)’:
       To see the code I am typechecking, use -ddump-deriv
     In the instance declaration for ‘Generic (Proxy t)’
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10775
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