On Feb 14, 11:10 pm, Chouser <chou...@gmail.com> wrote:
> On Sat, Feb 14, 2009 at 7:19 PM, Stephen C. Gilardi <squee...@mac.com> wrote:
>
>
>
> > "set" is a hash set. It will never contain two items with equal hashes.
>
> I don't think that's quite right. I don't think it matters in this
> case, but hash values aren't guaranteed unique. A hash-map can have
> two keys with the same hash value as long as = returns false. Vectors
> and lists with the same values evaluate as equal:
>
> user=> (= '(1 2) [1 2])
> true
>
Right, and therein lies the key to understanding the set behavior. A
set will contain at most one of any set of equal values. Equal objects
should have equivalent, but not necessarily unique, hash codes.
When you have polymorphic collections, as does Clojure, you need to
decide the type bounds of the equality test. If two collections needed
to be precisely the same type in order to be equal, the system would
be quite rigid, with lots of explicit copying and coercion.
Essentially, the polymorphism would be worthless. It ends up being
most flexible and useful to place the type bound as high up as
possible in the hierarchy, but not so high as to blur the core
distinctions between collection families. Clojure divides the
collections up into families for equality purposes - sequential, maps
and sets.
I can't overstate how important collection polymorphism is to the
design of Clojure.
One tradeoff is that sometimes you'd like explicit type to be part of
collection equality, but it would still beg the question as to where
to place the bounds, if other than exact match. For instance:
(count #{(hash-set 1) (sorted-set 1)}) === ???
they are both sets but not the same exact type.
Note that things are no different in Java, which also has polymorphic
collections:
(java.util.HashSet. [(java.util.ArrayList. [1 2])
(java.util.LinkedList. [1 2])])
-> #<HashSet [[1, 2]]>
(class (first *1))
-> java.util.ArrayList
Rich
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