Hi, all! It's been a while since I sent the message below to the GHC list, which I believe is its rightful forum, but I've had no reply so far. So I am giving the haskell list a try. Thanks in advance. Cheers, Jorge. --------------------------- Mensagem Original ---------------------------- Assunto: undecidable & overlapping instances: a bug? De: "Jorge Marques Pelizzoni" <jpeliz@icmc.usp.br> Data: Sab, Outubro 13, 2007 5:59 am Para: "GHC users" <glasgow-haskell-users@haskell.org> -------------------------------------------------------------------------- Hi, all! I am quite intrigued at the behaviour examplified in the attached module. It's true I am a newbie and probably don't quite get the whole consequence spectrum of -fallow-undecidable-instances, but why providing that dummy instance (commented out) get the thing to compile? By the way, I'm using GHC 6.6 on WinXP (actually, latest Visual Haskell with MS Visual Studio 2005) and the error message I get is: FooModule.hs:13:9: Could not deduce (Show a) from the context (Concrete a b) arising from use of `bar' at FooModule.hs:13:9-13 Possible fix: add (Show a) to the class or instance method `foo' In the expression: bar x In the definition of `foo': foo x = bar x In the definition for method `foo' A second question: which kinds of overlapping are covered by -fallow-overlapping-instances? It seems that the following (also in the attached module) is not allowed: instance (Show a, Abstract a b) => Concrete a b where foo x = show x instance (Abstract a b) => Concrete a b which gives me the message: FooModule.hs:17:0: Duplicate instance declarations: instance [overlap ok] (Show a, Abstract a b) => Concrete a b -- Defined at FooModule.hs:17:0 instance [overlap ok] (Abstract a b) => Concrete a b -- Defined at FooModule.hs:30:0 Thanks in advance for any pointers. Cheers, Jorge.