Here's why it would be useful to have the option of hash maps that use
identity...
Let's say my keys are rather long lists. Equality comparison will be
slow. But if I know that the keys are always the exact same long
lists (perhaps because I traversed the key sequence to find a key with
a certain property, and then am updating with that precise key as one
example), then I can save lots of time by using an identity
comparison.
FWIW, the Scheme I usually use (PLT Scheme) supports both eq? and
equal? based hash tables, and I have found both to be useful (although
I prefer equal? based hash tables as the default).
On Sat, Mar 7, 2009 at 7:57 AM, David Powell <djpow...@djpowell.net> wrote:
> What objects are you trying to store? The ideal in the Clojure world,
> is that most objects are immutable.
>
> Immutable objects will implement value equality in .equals() ('=' in
> Clojure), and you shouldn't care about whether two objects are
> identical? because it won't affect anything.
It's an efficiency issue.
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