On Tue, Jun 9, 2009 at 7:49 PM, Richard Newman<holyg...@gmail.com> wrote:
>
> Thanks, Konrad and Andrew, for chipping in!
>
>>> There's an outline of an implementation of multisets (I think that's
>>> the same as your bags) at:
>>>
>>> http://code.google.com/p/clojure-contrib/source/browse/trunk/src/
>>> clojure/contrib/types/examples.clj
>
> Thanks for the pointer.
>
> I apologize for the negativity, but these don't seem "first class" --
> that is, I can't treat them as I would sets:
>
> user=> (map identity #{:a :b :c})
> (:a :c :b)
> user=> (map identity (gc/conj (multiset {}) :a :b :c)))
> ([:c 1] [:b 1] [:a 1])
all that needs to happen is for multiset to provide it's own seq, like
it supplies it's own conj. seq would strip whatever information out.
you would need to call it explicitly on the multiset before passing it
to functions which call seq internally (map, filter, etc).
> The cardinality tracking has been (necessarily, I understand) lifted
> into the explicit contents of the data structure, so applications
> which call Clojure code that performs core operations (not your
> generic versions) on those structures will not work.
>
> Consequently, I couldn't use these with clojure.set's relational
> operations, for example:
>
> user=> (clojure.set/join
> #{{:name "John" :id 4} {:name "John" :id 3}}
> #{{:age 32 :id 4} {:age 19 :id 3}})
> #{{:age 19, :name "John", :id 3} {:age 32, :name "John", :id 4}}
>
> user=> (clojure.set/join
> (gc/conj (multiset {}) {:name "John" :id 4} {:name
> "John" :id 3})
> (gc/conj (multiset {}) {:age 32 :id 4} {:age 19 :id 3}))
> java.lang.ClassCastException: clojure.lang.PersistentArrayMap cannot
> be cast to java.util.Map$Entry (NO_SOURCE_FILE:0)
>
> So far as I understand it, to get 'native' multisets would require an
> implementation of IPersistentCollection, or perhaps IPersistentSet
> itself (if the contract of the interface doesn't impose set
> semantics), and ideally some Clojure surface syntax.
>
> PersistentTreeSet doesn't look too scary, so perhaps I'll dive in...
> more fun than my paid work, after all :)
>
>
>> Google Collections has Multisets - and they have an immutable
>> implementation.
>>
>> http://code.google.com/p/google-collections/
>
> Hmm, interesting. Not sure how their builder-based approach would fit
> in.
>
> For now, as I'm storing maps in my sets -- I'm doing relational stuff,
> after all -- I just add an :id entry in each map, filled by (iterate
> inc 0). Functionally the same as a multiset, so long as I ignore
> the :id 'column'...
>
> >
>
--
And what is good, Phaedrus,
And what is not good—
Need we ask anyone to tell us these things?
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