It feels like the Prelude insists on using [] and Maybe too much. I keep writing things like this: foo = foldl union emptySet $ maybe mzero return $ lookup pairs key goo = maybe emptySet toSomething $ lookup pairs key which really should look like this: foo = concat $ lookup pairs key goo = fmap toSomething $ lookup pairs key But, even if we don't have a Monadic/Functor Set, foo should at least be: foo = foldl union emptySet $ lookup key In other words, shouldn't Prelude define concat and lookup as: concat = foldr mplus mzero -- (Also, see PS) lookup key [] = mzero lookup key ((x,y):xyz) | key == x = return y | otherwise = lookup key xyz And if it is a fundamental problem adding constraints to instances, why not add all automatically derivable classes as constraints to all the Prelude classes (esp. Monad and Functor!) and automatically derive instances of all derivable classes unless the programmer defines his/own methods. -Alex- PS Shouldn't concat be defined with foldl and not foldr? Doesn't foldr imply that you can't concat infinite lists? (I know this is a FAQ, but where?) _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com