ocaml-lib-devel

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

[Richard J. <ri...@an...>]

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

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

[Richard J. <ri...@an...>]

On Tue, Apr 22, 2008 at 12:18:15PM -0700, Janne Hellsten wrote:
> Looks good, I would find this useful.

OK, I've committed find_map:
http://code.google.com/p/ocaml-extlib/source/detail?r=380

Rich.

-- 
Richard Jones
Red Hat

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

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

Hi Rich,

Would you also be so kind as to add a simple unit test for your new
function?  Although your addition is fairly straightforward, it's good
to test it in order to protect it in the future if someone wishes to
change it.

Janne

On Wed, Apr 23, 2008 at 5:05 AM, Richard Jones <ri...@an...> wrote:
> On Tue, Apr 22, 2008 at 12:18:15PM -0700, Janne Hellsten wrote:
>  > Looks good, I would find this useful.
>
>  OK, I've committed find_map:
>  http://code.google.com/p/ocaml-extlib/source/detail?r=380
>
>  Rich.
>
>  --
>  Richard Jones
>  Red Hat
>

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

[Richard J. <ri...@an...>]

On Wed, Apr 23, 2008 at 10:50:17AM -0700, Janne Hellsten wrote:
> Hi Rich,
> 
> Would you also be so kind as to add a simple unit test for your new
> function?  Although your addition is fairly straightforward, it's good
> to test it in order to protect it in the future if someone wishes to
> change it.

Done (rev 381).

Rich.

-- 
Richard Jones
Red Hat

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

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

That was quick, thanks!

Janne

On Wed, Apr 23, 2008 at 11:09 AM, Richard Jones <ri...@an...> wrote:
> On Wed, Apr 23, 2008 at 10:50:17AM -0700, Janne Hellsten wrote:
>  > Hi Rich,
>  >
>  > Would you also be so kind as to add a simple unit test for your new
>  > function?  Although your addition is fairly straightforward, it's good
>  > to test it in order to protect it in the future if someone wishes to
>  > change it.
>
>  Done (rev 381).
>
>
>
>  Rich.
>
>  --
>  Richard Jones
>  Red Hat
>