Following a suggestion from Cale Gibbard I added a convenient feature to Djinn. You can now define type classes and have contexts on functions. As with Djinn in general there is no polymorphic instantiation, so the methods of a class must not mention any type variables, but the class parameters. This makes the whole thing rather limited, but it was a quick hack. Sample session

Welcome to Djinn version 2007-04-20.
Type :h to get help.
Djinn> f ? (Eq a) => a -> a -> (b,c) -> Either b c
f :: (Eq a) => a -> a -> (b, c) -> Either b c
f a b =
    case a == b of
    False -> \ (c, _) -> Left c
    True -> \ (_, d) -> Right d

The Djinn source is at darcs.augustsson.net/Darcs/Djinn as usual. Thanks Cale!