I seem to be a relatively lone voice thinking it makes sense to preserve
the type of the map, and honestly, I'm not going to lose any sleep if this
patch is never actually implemented, but here's my two cents on the
"optimal" design for records.
Earlier in the thread, I mentioned two mental models about how select-keys
works. Even though the actual implementation involves starting from an
empty map and building back up, I think the behavior of select-keys should
mimic the mental model of discarding irrelevant keys.
The two mental models predict somewhat differently what would happen when
working with records. Records can hold additional keys. So imagine
someone dumps a whole lot of additional keys, and then wants to "recover"
the original, true record, by calling select-keys with the original keys in
the record. I think this is a use case worth supporting. This corresponds
to the mental model of discarding unwanted keys.
To implement this for records:
If any of the original record keys do not appear in the selected keys,
then, as with dissoc, we must revert back to a map as there is no valid way
to represent this as a record.
Otherwise, maintain the record type -- the core record contents are
preserved, and essentially you only need to call select-keys only on the
extended map of extra keys stored in the record.
On Sat, Nov 2, 2013 at 8:49 AM, Andy Fingerhut <andy.finger...@gmail.com>wrote:
> I attached another patch to the ticket. It builds up the answer from the
> empty map {} if the argument is a record (as the current select-keys does),
> but from (empty map) if it is not a record, so it will preserve sortedness
> of the argument. Not sure if there are any other cases that are a problem,
> or if it does what everyone would expect, but it should be closer.
>
> http://dev.clojure.org/jira/browse/CLJ-1287
>
> Andy
>
>
> On Sat, Nov 2, 2013 at 6:07 AM, Alex Miller <a...@puredanger.com> wrote:
>
>> > One other point:
>> > Sometimes people use sorted maps and array maps specifically for
>> scenarios in which the keys are not hashable and therefore hash maps would
>> not apply. Dumping the contents into a regular map in such cases doesn't
>> make much sense.
>>
>> Everything is hashable, not sure what a non-hashable key means. Array
>> maps use the hash of the key to determine the array bucket. If you get the
>> hash code of a sorted map, it will get the hash of all keys and values.
>>
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