OK, so since noone liked my original example here's another one. It involves no defaulting and no classes in the funny function definition. -- Here's the type signature that makes a difference. --fun :: a -> Char fun x = const (fun x) (fun True) fix f = let x = f x in x class C a where m :: a -> String instance C Char where m _ = "has signature" instance C Bool where m _ = "no signature" main = putStrLn (m (fix fun)) It is not at all surprising that you can write this. Originally type signatures only allowed you to put a signature that was more specific. Polymorhic recursion on the other hand allows you to make the type more general by putting a type signature on a definition. Combining these you can make the signature be incomparable to the deduced type. Using the class system you can then dispatch on the type and get different behaviour. -- Lennart