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])
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'...
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