[plt-scheme] Is there a set data-type?

On Fri, Feb 05, 2010 at 10:05:29AM -0600, Robby Findler wrote:
> On Fri, Feb 5, 2010 at 10:03 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:
> > Robby Findler wrote:
> >>
> >> On Fri, Feb 5, 2010 at 9:54 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:
> >>>
> >>> I guess the characteristic function approach involves using lambdas to
> >>> represent a set... :-/ I am worried with efficiency but I will give it a
> >>> try.
> >>
> >> FWIW, there are some sets that are far more efficiently represented
> >> using functions than other data structures if you can write the
> >> function directly (eg, (lambda (x) (< 0 x 100000)) is far better than
> >> checking membership in a 100000 element list), but when you pile up a
> >> series of unions and intersections and things like that you end up
> >> getting something like linear work in the original construction of the
> >> set. If you want to do better maybe the right thing is to considering
> >> developing a little PL and interpreting it (with special purpose
> >> optimizations in there for the things you expect to happen
> >> frequently).
> >>
> >> Just a thought, of course. :)
> >>
> >
> > Thanks for the thoughts on performance.
> > Another thing I will have to think about is computing the cardinality of the
> > set which might return +inf.0 of course. I am not even sure this
> > representation allows the computation of cardinality in a finite amount of
> > time.
> 
> You could use objects that have to implement various methods, member?,
> cardinality, union, enumerate (to debug; for infinite sets it would
> just stop after a while).

For inumeration, you'd have a function that would return an element and 
the rest of the set.  Rather like car, cdr, and null? for lists.

> 
> Robby
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