[GHC] #11562: WARNING: file compiler/stgSyn/CoreToStg.hs, line 250: $fCategoryConstraint:- True False

9 Feb 2016

      #11562: WARNING: file compiler/stgSyn/CoreToStg.hs, line 250:
$fCategoryConstraint:- True False
-------------------------------------+-------------------------------------
           Reporter:  slyfox         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           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:
-------------------------------------+-------------------------------------
 Tried to debug unrelated build failure of constraints-0.4.1.3
 and found this in today's ghc-HEAD:

 {{{
 $ inplace/bin/ghc-stage2 -fforce-recomp -c C.hs -O0
 WARNING: file compiler/stgSyn/CoreToStg.hs, line 250
   $fCategoryConstraint:- True False
 }}}

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE Rank2Types #-}
 {-# LANGUAGE GADTs #-}

 module C () where
 import Control.Category
 import GHC.Types (Constraint)

 data Dict :: Constraint -> * where
   Dict :: a => Dict a

 infixr 9 :-

 newtype a :- b = Sub (a => Dict b)

 instance Category (:-) where
   id  = refl
   (.) = trans

 infixl 1 \\

 (\\) :: a => (b => r) -> (a :- b) -> r
 r \\ Sub Dict = r

 trans :: (b :- c) -> (a :- b) -> a :- c
 trans f g = Sub (Dict \\ f \\ g)

 refl :: a :- a
 refl = Sub Dict

 {-
 $ inplace/bin/ghc-stage2 -fforce-recomp -c C.hs -O0

 WARNING: file compiler/stgSyn/CoreToStg.hs, line 250
   $fCategoryConstraint:- True False
 -}
 }}}

 Original file it was factored out is:
 https://github.com/ekmett/constraints/blob/c550b7653e88d58882873d11f05538d78...

 Might be a result of Phab:D1889

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11562
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