On Mon, Dec 20, 2010 at 2:49 AM, Alex Osborne <a...@meshy.org> wrote:
> Ken Wesson <kwess...@gmail.com> writes:
>
>> Ah. So, like the confused situations you get with Java's mutable
>> collections. Two lists are equal if they have the same contents in the
>> same order -- but then you use one as a key in a hashmap, and then add
>> an item to it, and boom! Clojure separates this stuff out because the
>> Clojure vector's immutability makes its value stable given its
>> identity. Refs and atoms and agents can encapsulate mutable state, but
>> their identity (as defined by = and hash) is fixed rather than
>> changing with its state.
>
> Sort of. Identity (in the Clojure model) is not the same concept as
> equality. Nor is it reference equality ("identical?"). The overloading
> of terminology is somewhat unfortunate.
>
> "By identity I mean a stable logical entity associated with a series
> of different values over time." -- clojure.org/state
>
> As Laurent mentioned the usual identities in Clojure are reference
> objects: vars, atoms, refs and so on.
>
>> And some objects (keywords and symbols) exist
>> to be almost pure identity, used to label other things.
>
> Symbols and keywords (and database IDs) aren't identities, they're
> identifiers (names).
It seems you're using "identity" a little bit oddly here. In ordinary
usage, "identity" would indeed be close to synonymous with
"identifier"; the way you're using it here is actually closer to the
usual comp-sci concept of a "variable": a holder of mutable state,
which can be pointed to a succession of different individual states.
So part of this is a confusion arising from slightly odd or
idiosyncratic terminology.
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