[GHC] #9559: "Illegal equational constraint" that might not be the users fault

[GHC]

#9559: "Illegal equational constraint" that might not be the users fault
-------------------------------------+-------------------------------------
       Reporter:  rrnewton           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 If you try the latest cabal head and GHC together as of 2014.09.04, and
 then you try to install statistics, you'll see an error like the
 following, which does not occur in GHC 7.8.3:

 {{{
 cabal-1.21 install -w ghc-7.9.20140904
 Resolving dependencies...
 Configuring statistics-0.13.2.1...
 Building statistics-0.13.2.1...
 Preprocessing library statistics-0.13.2.1...
 [ 1 of 46] Compiling Statistics.Test.Internal (
 Statistics/Test/Internal.hs, dist/build/Statistics/Test/Internal.o )
 [ 2 of 46] Compiling Statistics.Test.Types ( Statistics/Test/Types.hs,
 dist/build/Statistics/Test/Types.o )
 [ 3 of 46] Compiling Statistics.Internal ( Statistics/Internal.hs,
 dist/build/Statistics/Internal.o )
 [ 4 of 46] Compiling Statistics.Transform ( Statistics/Transform.hs,
 dist/build/Statistics/Transform.o )

 Statistics/Transform.hs:113:3:
     Illegal equational constraint Control.Monad.Primitive.PrimState m
                                   ~ s
     (Use GADTs or TypeFamilies to permit this)
     When checking that ‘bitReverse’ has the inferred type
       bitReverse :: forall (m :: * -> *) b.
                     (Control.Monad.Primitive.PrimMonad m,
                      Control.Monad.Primitive.PrimState m ~ s) =>
                     Int -> Int -> m b
     In an equation for ‘mfft’:
 }}}

 (Indeed, adding the GADTs language pragma makes the type checker happy, as
 in the commit here:
 https://github.com/rrnewton/criterion/commit/3a2487176e92b6972560ab1abdfa09a2b3eb4a49
 )

 Hmm, well, the source code in this case did not explicitly add the "~ s"
 constraint, the inference cooked it up.  Is this a GHC bug in 7.9 or in
 7.8?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9559>
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