fold_left f lst start leftfolds the function f over the list lst starting with the value start. Continuation of the fold is determined by the bind operator >>= of the monad M. E.g. if the monad M is Option the folding stops as soon as f e acc returns the value None.

     fold_left f [a b c ...] s =
       M.(f a s   >>= fun acc ->
          f b acc >>= fun acc ->
          f c acc >>= fun acc ->
          ...)

The same as fold_left just right folding.

     fold_right f [... x y z] s =
     fold_left f (rev [... x y z]) s =
       M.(f z s   >>= fun acc ->
          f y acc >>= fun acc ->
          f x acc >>= fun acc ->
          ...)

The same as fold_left except that the folding function receives the position of the first argument in the list as an additional argument.