Wolfgang Jeltsch wrote:

Not quite, I'm actually not sure if I want this, I just noted that it was possible. :) As David Menendez pointed out, empty classes probably aren't that useless.

Anyway, not treating empty classes specially and just making the universal instance explicit may be enough. Foo is declared an alias for Bar by writing:

class Foo a => Bar a instance Foo a => Bar a

The latter is not Haskell98, but a harmless extension. Doing it this way "feels better" than introducing new syntax and semantics, at least to me.

Udo.