I have two typeclasses, Foo and Bar, with some instances, defined as such: module Test where class Foo a where foo :: a -> String class Bar a where bar :: a -> String instance Foo Int where foo = show instance Bar Int where bar a = (show a) ++ " bar!" instance Foo Char where foo = show instance Foo a => Bar a where bar = foo If I try to compile this via GHC it complains loudly unless I feed it "-fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances", in which case it seems to do the right thing. However, the warnings (and the names of the flags - "undecidable instances" doesn't sound good) make me nervous, and confused. Why is there a problem with saying "every instance of Foo is also an instance of Bar, and here's how"? Abe