While trying out the following example, in an attempt to learn something about the fiddly case where a type class instance tries to use an instance that is more specific than itself module Testing where class Foo a where { bar :: a -> Int } instance Foo Int where bar i = i instance Foo a => Foo [a] where bar [] = bar [1] bar ([x]) = 1 bar (x:xs) = (bar x) + (bar xs) It is kind of like polymorphic recursion, I suppose. I get the following exception [1 of 1] Compiling Testing ( strange.hs, interpreted ) *** Exception: typecheck/TcEnv.lhs:(365,0)-(392,32): Non-exhaustive patterns in function find_thing Is the example supposed to work? I'm trying to determine the source of a problem with type classes and GADTs and I figured this example using "normal" data types would be a good place to start in understanding what was going wrong. I was using ghci version 6.5.20060503.