INTERNAL ERROR: findBtyvsInt strikes again!

[Alexandru D. Salcianu]

Hello!

I'm using Hugs (the November 2002 version) and I've encountered the
"INTERNAL ERROR: findBtyvsInt" error.  I've read online that this
error was known in the November 1999 version, but was fixed in the
2000 version.  I'm using the 2002 version and still get it.

I've encountered the error while working on a collection library (of
course, I'm using the multi-parameter typeclass extension).

$ hugs -98
__   __ __  __  ____   ___      _________________________________________
||   || ||  || ||  || ||__      Hugs 98: Based on the Haskell 98 standard
||___|| ||__|| ||__||  __||     Copyright (c) 1994-2002
||---||         ___||           World Wide Web: http://haskell.org/hugs
||   ||                         Report bugs to: hugs-bugs at haskell.org
||   || Version: November 2002  _________________________________________

Hugs mode: Restart with command line option +98 for Haskell 98 mode

Reading file "/usr/lib/hugs/lib/Prelude.hs":
                   
Hugs session for:
/usr/lib/hugs/lib/Prelude.hs
Type :? for help
Prelude> :load Map.hs
Reading file "Map.hs":
Type checking      
INTERNAL ERROR: findBtyvsInt
Prelude> :version
-- Hugs Version November 2002

Here is the content of Map.hs:

module Map where

-- objects of type ce represent sets of e's
class (Eq e) => Set e ce | ce -> e where
    equalSet  :: ce -> ce -> Bool

-- objects of type map represent maps from key to value
class (Eq key) => FiniteMap key value map | map -> key value where
    -- returns all keys from map, as a set of type keyset
    mapKeys     :: (Set key keyset) => map -> keyset

    equalMap    :: map -> map -> Bool
    {-- ERROR in this incomplete default implem. of equalMap --}
    equalMap m1 m2 = 
        let 
            (keys1 :: [key]) = (mapKeys m1)
            (keys2 :: [key]) = (mapKeys m2)
        in
            equalSet keys1 keys2

instance (Eq a) => Set a [a] where
    equalSet = (==)


(I've trimmed the code down to obtain a small bug report; the above
code should be enough to trigger the error).  This code compiles fine
with the GHC 6 (with the command line "ghc -c -fglasgow-exts Map.hs").

Best,

Alex