I'm trying to achieve something like (ocaml) let rec zipfold f init it1 it2 = if it1#is_end || it2#is_end then init else let v1, v2 = it1#value, it2#value in it1#next ; it2#next ; zipfold f (f v1 v2 init) it1 it2 which is a foldl on 2 iterators. I've managed to use monads for one iterator, but i completly miss the way to work with 2 monads. Is there really no other solution than creating a monad over the 2 iterators?? -- using the StateTrans as in http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm newtype StateTrans s a = ST( s -> (s, a) ) -- can't write l == [] otherwise it need Eq on elem of lists is_empty_list [] = True is_empty_list _ = False is_empty = ST(\l-> (l, is_empty_list l)) val = ST(\l -> (l, head l)) next = ST(\l -> (tail l, ())) myfoldl f init = do b <- is_empty if b then return init else do e <- val next myfoldl f (f e init) thanks -- Pixel