The following module is rejected by both ghc -fglasgow-exts -fallow-undecidable-instances and hugs -98 ------------------------------------------------------------------------ class HasFoo a foo | a -> foo where foo :: a -> foo data A = A Int data B = B A instance HasFoo A Int where foo (A x) = x instance HasFoo A foo => HasFoo B foo where foo (B a) = foo a ------------------------------------------------------------------------ The error messsage says that the type inferred for foo in B's instance is not general enough: the rhs has type "HasFoo B Int => B -> Int", but "HasFoo B foo => B -> foo" was expected. Should it really be wrong? I don't know the details of type inference with fundeps, but intuitively it should work, yielding an instance HasFoo B Int. Could it be made legal please? With the fundep removed, it works. I need it for a preprocessor which generates instances like that for B without knowing the type to put as the second class argument. Fundeps aren't essential, but... -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK