type aliases in instances

In Haskell type aliases are not allowed in instance definitions. Is there a particular reason for this? For example, is there a problem with type inference? I noticed that when composing monads (in the Moggi/Wadler style) one easily ends up with a cascade of coercions enforced by datatype declarations. Looks a bit tedious. This can be avoided by using type aliases but then the monads in use cannot be instances of the Monad class. Phil uses aliases in his "Essence of functional programming" paper too. But not declaring the monads to be in class Monad can hardly be good style, can it? Many thanks for your help in advance. Bernhard --- Bernhard Reus, COGS University of Sussex