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@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