Jason, this was very helpful. sorted-map's correct behavior compared
to the naive behavior of hash-map is what confused me. Thanks for
clearing this up.
On May 21, 6:31 am, Jason Wolfe <jawo...@berkeley.edu> wrote:
> The crux of the issue:
>
> user> (= (long 1) (int 1))
> true
> user> (.equals (long 1) (int 1))
> false
> user> (.equals (int 1) 1)
> true
> user> (.equals (long 1) 1)
> false
>
> So, as you can see, small integer literals are Integers by default in
> Clojure. And, while Integers and Longs with the same value are
> Clojure "=", they are not ".equal" to each other in Java-land.
>
> Now, since Clojure's maps implement java.util.Map, they're supposed to
> use java's .equals. (They must do so to use existing .hashCode methods
> on Java objects). So, you get:
>
> user> ({ (long 1) :found} (long 1))
> :found
> user> ({ (long 1) :found} 1)
> nil
>
> (If you want to lookup from your map with Long keys, you better make
> sure the lookup keys are Longs too). Be careful to keep these
> straight, a map can have distinct k-v pairs for different numeric key
> types:
>
> user> ({ (long 1) :found-a-long (int 1) :found-an-int} 1)
> :found-an-int
>
> Finally, the situation with sorted-map is a bit strange. For example,
> see the third paragraph of:
>
> http://java.sun.com/j2se/1.4.2/docs/api/java/util/TreeMap.html
>
> Now, Clojure's default comparator considers (long 1) and (int 1) to be
> equal (see "(doc compare)"):
>
> user> (compare (long 1) (int 1))
> 0
>
> and so by default sorted-map does too. Note that the caveats
> mentioned in the above webpage apply; technically, sorted-map with the
> default comparator does not correctly implement the java.util.Map
> interface, although in practice this may lead to more sensible-seeming
> behavior.
>
> Hopefully this helps, and I haven't misunderstood/misprepresented what
> Clojure does here...
>
> -Jason
>
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