clojure.set/intersection is documented to work on "input sets". In Clojure,
all sets are finite. (range) is a lazy sequence, which isn't a set. You could
attempt to make a set out of it with (set (range)), but that won't work because
it will try to consume the entire unending sequence.
I would guess that extending clojure.set/intersection to work on lazy sequences
as well, possibly infinite, would be considered out of scope for what it is
intended to do, but that is only my guess.
You are of course welcome to make your own generalized version of intersection
that works as you describe.
Andy
On Feb 6, 2013, at 8:42 AM, Andrea Chiavazza wrote:
> Currently this hangs, makes my machine quickly run out of memory and swap.
>
> (clojure.set/intersection #{1 2} (range))
>
> The problem seems to be that count is called on both arguments to find the
> smallest set.
> Finding the shortest of 2 seqs lazily shouldn't be a problem, but another
> problem I can spot would be that contains? should be used only for some data
> structures, like hash-set, otherwise I guess "some" could be used, which
> would do the right thing, like in:
>
> (some #{1} (range))
> 1
> (some #{2} (range))
> 2
> At this point execution will stop and return the #{1 2} as all elements of
> the smallest set have been tested.
>
> Would it be possible and worth the added complication to fix this ?
> There could also be some potential performance improvements, for example
> removing count might be slightly more efficient when big sets are involved.
>
> I came up with this while trying to solve 4clojure #108 "find the smallest
> single number which appears in all of the sequences".
> My approach to the 4clojure problem might be wrong altogether, yet I can
> imagine removing this limitation could be useful for some real problems too.
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