Re: [Ocaml-lib-devel] Map with set operations over the keys

[Martin J. <mar...@em...>]

On Mon, 6 Jun 2005, Nicolas Cannasse wrote:

> > > I would say it's bad API design to use a "merge" function in order to
> remove
> > > elements.
> > > A more specific function should be provided.
> >
> > I do not think so. When your merge gives accidentally 0, and the value
> > should be removed, you do not want to scan the map once more to remove
> > the zero ...
>
> Then it's no longer a merge...
> What you want is a more generic function that will do a merge+filter.

"reduce" as suggested by Florian seems to be appropriate.

The following signatures seem good to me:

(* Exception Remove can be raised by the user-given functions and means
   "do not keep a binding to this key" for both union and inter. *)

val union :
  ('key -> 'a -> 'c) -> ('key -> 'a -> 'b -> 'c) -> ('key -> 'b -> 'c) ->
  ('key, 'a) t -> ('key, 'b) t -> ('key, 'c) t

val inter :
  ('key -> 'a -> 'b -> 'c) -> ('key, 'a) t -> ('key, 'b) t -> ('key, 'c) t

val diff :
  ('key, 'a) t -> ('key, 'b) t -> ('key, 'a) t


Plus maybe some specialized versions for efficiency and/or convenience.

I am not sure if the gain in performance would be terrific, and
specialized versions of union and inter can easily be defined out of PMap
anyway, so I wouldn't insist too much on having those functions or not.


Martin

--
Martin Jambon, PhD
http://martin.jambon.free.fr