Opened 4 years ago

Closed 4 years ago

#9560 closed bug (duplicate)

"Illegal equational constraint" that might not be the users fault

Reported by: rrnewton Owned by:
Priority: normal Milestone:
Component: Compiler Version: 7.9
Keywords: Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


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-
Building statistics-
Preprocessing library statistics-
[ 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 )

    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: )

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?

Change History (1)

comment:1 Changed 4 years ago by monoidal

Resolution: duplicate
Status: newclosed

Double submission: #9558.

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