well, what we need is (unit :: a -> m a) and (zero :: m a). Maybe gives us that nicely (Just,Nothing). Monad is not nice, (return,fail "..."). MonadPlus has (return,mzero) BUT is too specific, as some types with a (unit,zero) aren't even Monads, or don't have mplus. Alternative (of Applicative fame) has (pure,empty) and no "fail" to be confused with... but it's less used yet, and it still requires fmap, ap and mplus (er, I mean, <*> and <|>). Do we want every return type that's simple pattern-matchable data to be a type-class instead, so that we can have fewer explicit conversions applied to them? (I suppose this is considered explicit deforestation in the sense that ndm says GHC doesn't know how to do with returned "data") I was assuming that we don't want that, and that therefore I prefer Maybe. But I'm not sure anymore. It seems that can give conciseness without harm... except for making it harder to tell what type everything is (when looking a text-editor that doesn't know Haskell type inference). -Isaac