[Ocaml-lib-devel] Re: Extra function for List

[Christophe P. <chr...@gm...>]

Hello,

I have looked inside extList.ml and noticed the hackery used to forgo
List.rev to keep functions tail-recursive.  Anyways, here is a new fold_map
that uses the same hackery:

let fold_map f b l =3D
    let rec loop x dst =3D function
        | [] -> x
        | h :: t ->
      let (x', h') =3D f x h in
      let r =3D {hd =3D h'; tl =3D []} in
      dst.tl <- inj r;
      loop x' r t
    in
    let dummy =3D dummy_node() in
    let e =3D loop b dummy l in
    (e, dummy.tl)

cheers,
Christophe

On 12/6/05, Christophe Poucet <chr...@gm...> wrote:
>
> Hello,
> I have here a function I would like to propose for the list module.  It's
> a foldmap.  Basically it's a map but it allows you to pass along a variab=
le
> among the different calls.  Perhaps it could be called foldmap_left such
> that a foldmap_right could be implemented as well.
>
> let foldmap f b l =3D
>   let (e, l') =3D List.fold_left
>    (fun (x, t) a -> let (x', a') =3D f x a in (x', a'::t)) (b, []) l
>   in (e, List.rev l')
>
>
> An example of it's use:
> foldmap (fun a e -> (a+1, e)) 0 [1;2;3] =3D> (3, [1;2;3])
>
> The motivation for this is that sometimes you want to map a certain list
> (imagine for instance a list of statements in a compiler), but you also w=
ant
> to gradually build up a functional environment as you pass down this list=
.
>
> Cheers,
> Christophe
>