On Dec 2, 12:06 pm, ataggart <alex.tagg...@gmail.com> wrote:
> On Dec 2, 7:10 am, Stefan Kamphausen <ska2...@googlemail.com> wrote:
>
>
>
>
>
> > Hi,
>
> > while studying the collection types and trying to find out which
> > functions work on all collection types (i.e. lists, vectors, maps,
> > sets) I was flabbergasted by the following
>
> > user> (contains? (list 1 2 3) 3)
> > false
>
> > OK, the doc of contains? told me that for indexed collection-types it
> > will only check, whether the index is within the valid range. So
> > maybe:
>
> > user> (contains? (list 1 2 3) 1)
> > false
>
> > At that point I dived into the implementation and found that in
> > RT.java checks for the type of collection passed but leaves out
> > PersistenList.
>
> > I'd like to understand the (probably well-grounded) reason for that.
> > As far as I can see PersistentList extends Counted, so the check for
> > the index-range should at least be possible. However, I think people
> > would expect an equality check in the background, which OTOH would be O
> > (N), probably.
>
> > Kind regards,
> > Stefan
>
> That's because contains? checks for keys not values. Since a list
> isn't associative (unlike vectors and maps), it doesn't have keys,
> thus contains? doesn't apply. I too was first thrown off by assuming
> contains? worked on the values of a collection; for that you can
> instead use some.
>
> (if (some #(= :a) '(:a :b :c))
> :contains-a
> :does-not-contain-a
Erm... that predicate should be #(= :a %) or (partial = :a) or #{:a}.
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