Re: [Ocaml-lib-devel] Idea with ExtList.fold_right

[Nicolas C. <war...@fr...>]

> > Ok, so let's provide them a "list only" version of fold_right (
ExtList.fold
> > ? ) that will do the job in a tail-rec way using internals set_cdr to
> > construct the list in the user-wanted way. This of course does not cover
all
> > the cases, but should be enough to preserve speed where it is needed
(btw
> > enums should be even better for this kind of things ^^;) . The
difference of
> > this fold is that we are iterating from left_to_right but I don't think
that
> > a lot of users actually wants to do so.
>
> Well, we already have List.map to convert a 'a list to a 'b list and keep
> the order the same.

List.map is not the same as List.fold !
- with fold, you can "skip" elements in a list, doing then a filter_map
- you can access the accumulator , and this is useful in many cases (no
duplicates for example)

I suggest having something like :

let fold f l =
    let x = [ Obj.magic () ] in
    let rec loop dst = function
        | [] -> ()
        | h :: t ->
            let r = [ f h (List.tl x) ] in
            Obj.set_field (Obj.repr dst) 1 (Obj.repr r);
            loop r t
    in
    loop x;
    List.tl x

Nicolas Cannasse