Hello, I have send a message to these lists a weak ago with almost the same topic, but I have now additional observations: The problem is not related to option -O in GHC 6.2.1 ! We use in conjunction these command line switches for the compilation: --make -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances and get this error message, when we try to compile the complete project: Compiling Common.Lib.Rel ( ./Common/Lib/Rel.hs, ./Common/Lib/Rel.o ) ./Common/Lib/Rel.hs:102: Could not unambiguously deduce (Show (a, a)) from the context (Show (Rel a), Show a, Ord a) arising from use of `show' at ./Common/Lib/Rel.hs:102 The choice of (overlapping) instance declaration depends on the instantiation of `a' Probable fix: Add (Show (a, a)) to the class or instance method `show' Or add an instance declaration for (Show (a, a)) In the first argument of `(.)', namely `show' In the definition of `show': show = show . (Common.Lib.Set.fromList . toList) In the definition for method `show' When we issue the same ghc call again, Common/Lib/Rel is compiled without errors. Well, the needed tar-ball (4.2M) to reproduce the error is downloadable at http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/CoFI/hets... Cheers, Klaus -- Klaus Lüttich Email: luettich@tzi.de FB 3 Phone: +49-421-218-7129 Uni Bremen