[GHC] #12399: DeriveFunctor fail

16 Jul 2016

      #12399: DeriveFunctor fail
-------------------------------------+-------------------------------------
           Reporter:  osa1           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           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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 {{{#!haskell
 {-# LANGUAGE DeriveFunctor, MagicHash, UnboxedTuples #-}

 module Lib where

 import GHC.Exts

 newtype RmLoopsM a = RmLoopsM { runRmLoops :: Int# -> (# Int#, a #) }
 }}}

 Functor instance for this can be derived like this:

 {{{#!haskell
 instance Functor RmLoopsM where
   fmap f (RmLoopsM m) = RmLoopsM $ \i -> case m i of
                                            (# i', r #) -> (# i', f r #)
 }}}

 `DeriveFunctor` instead generates something like this:

 {{{#!haskell
 instance Functor RmLoopsM where
   fmap f_a2Oh (Lib.RmLoopsM a1_a2Oi)
     = RmLoopsM
         ((\ b6_a2Oj b7_a2Ok
             -> (\ b5_a2Ol
                   -> case b5_a2Ol of {
                        ((#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op)
                          -> (#,#)
                               ((\ b2_a2Oq -> b2_a2Oq) a1_a2Om)
                               ((\ b3_a2Or -> b3_a2Or) a2_a2On)
                               ((\ b4_a2Os -> b4_a2Os) a3_a2Oo)
                               (f_a2Oh a4_a2Op) })
                  (b6_a2Oj ((\ b1_a2Ot -> b1_a2Ot) b7_a2Ok)))
            a1_a2Oi)
 }}}

 which fails with

 {{{
 Main.hs:17:25: error:
     • The constructor ‘(#,#)’ should have 2 arguments, but has been given
 4
     • In the pattern: (#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op
       In a case alternative:
           ((#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op)
             -> (#,#)
                  ((\ b2_a2Oq -> b2_a2Oq) a1_a2Om)
                  ((\ b3_a2Or -> b3_a2Or) a2_a2On)
                  ((\ b4_a2Os -> b4_a2Os) a3_a2Oo)
                  (f_a2Oh a4_a2Op)
       In the expression:
         case b5_a2Ol of {
           ((#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op)
             -> (#,#)
                  ((\ b2_a2Oq -> b2_a2Oq) a1_a2Om)
                  ((\ b3_a2Or -> b3_a2Or) a2_a2On)
                  ((\ b4_a2Os -> b4_a2Os) a3_a2Oo)
                  (f_a2Oh a4_a2Op) }
 }}}

 I think it's supposed to ignore RuntimeRep args during the fold
 (`TcGenDeriv.functorLikeTraverse`).

 Tried with: HEAD, 8.0.1.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12399
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