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

[Brian H. <bri...@ql...>]

On Tue, 17 Jun 2003, Nicolas Cannasse wrote:

> 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)

Point.

> 
> 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

Maybe combine this with filter_map-like capabilities?  Something like:

let foo f l =
    let x = [ Obj.magic () ] in
    let rec loop dst = function
        | [] -> ()
        | h :: t ->
           match f h (List.tl x) with
               None -> loop dst t
               | Some x -> 
                   let r = [ x ] in 
                   Obj.set_field (Obj.repr dst) 1 (Obj.repr r);
                   loop r t
    in
    loop x lst;
    List.tl x

Brian