Luke Palmer wrote:
Isn't a type which is both a Monad and a Comonad just Identity?
(I'm actually not sure, I'm just conjecting)
Good idea, but it's not the case. data L a = One a | Cons a (L a) -- non-empty list -- standard list monad instance Monad L where return = One -- join concatenates all lists in the list join (One y) = y join (Cons (One x) ys) = Cons x (join ys) join (Cons (Cons x xs) ys) = Cons x (join (Cons xs ys)) class Comonad c where counit :: c a -> a cobind :: c a -> (c a -> b) -> c b instance Comonad L where counit (One x) = x -- that's why we insist on non-emptiness counit (Cons x _) = x -- f has access to the whole past cobind ys@(One x) f = One (f ys) cobind ys@(Cons x xs) f = Cons (f ys) (cobind f xs) Checking the laws is left as an exercise for the reader :) Regards, apfelmus