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

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

On Mon, 6 Jun 2005, Christophe Raffalli wrote:

> Nicolas Cannasse wrote:
> >>>After a second though I think it's little overweighted to make the user
> >>>specify a merge function. Why not simply specify that in such cases, the
> >>>item of the first map is selected ? That seems more reasonable.
> >>>
> >>
> >>No, you can for instance implement multiset as map to natural ... and
> >>the merge function is then addition.
> >>
> >>Your merge function should even raise the exception Remove when you want
> >>to say that the result should be removed from the map ... (For insance,
> >>if you have Map to integer and want to say that 0 should be removed from
> >>the map).
> >
> >
> > 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 ...

I concur. Maybe we can use another name than "merge", but I like this use
of exceptions. Here the term "exception" is really appropriate since it's
an exception to the mathematical meaning of union (or intersection).

Martin

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