Re: [Ocaml-lib-devel] Proposed higher-order function: List.find_map

[Janne H. <jjh...@gm...>]

Looks good, I would find this useful.

Janne

On Tue, Apr 22, 2008 at 3:45 AM, Richard Jones <ri...@an...> wrote:

> I'd like to propose the following function for inclusion in ExtList:
>
>  (**
>       [find_map pred list] finds the first element of [list] for which
>       [pred element] returns [Some r].  It returns [r] immediately
>       once found.
>
>       @raise Not_found if no element matches the predicate.
>
>       See also {!filter_map}.
>    *)
>
>  val find_map : ('a -> 'b option) -> 'a list -> 'b
>
>  let rec find_map f = function
>    | [] -> raise Not_found
>    | x :: xs ->
>        match f x with
>        | Some y -> y
>        | None -> find_map f xs
>
> The HOF is analogous to filter_map, but only returns the first element
> found, rather than all matching elements.
>
> I find this HOF useful when iterating over XML [1].  For example if
> you have XML like this:
>
>  <guest>
>    <os_type>hvm</os_type>
>    <arch name='ppc'/>
>  </guest>
>
> and you want to find the <os_type> content, then assuming 'guest' is a
> variable containing the child nodes of <guest>:
>
>  let os_type =
>    try
>      find_map (
>          function
>          | Element ("os_type", _, [PCData "hvm"]) -> Some VT_HVM
>          | Element ("os_type", _, [PCData "xen"]) -> Some VT_Xen
>          | _ -> None
>      ) guest
>    with
>      Not_found -> VT_Unknown in
>
> If nobody objects I would like to add this to extlib.
>
> Rich.
>
> [1] I know it'd be better to do this with CDuce, but that would
> require there to be some sort of schema to the XML which doesn't
> change with the phase of the moon.
>
> --
> Richard Jones
> Red Hat
>
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