Set length property

Tab Atkins Jr. jackalmage at
Tue Feb 14 13:20:31 PST 2012

On Tue, Feb 14, 2012 at 1:10 PM, Dean Landolt <dean at> wrote:
> On Tue, Feb 14, 2012 at 3:54 PM, Tab Atkins Jr. <jackalmage at>
> wrote:
>> On Sun, Feb 12, 2012 at 7:08 PM, Brendan Eich <brendan at> wrote:
>> > See the thread containing Dean Landolt's dissent on 'length' being the
>> > best
>> > name:
>> >
>> >
>> >
>> > The January 19 2012 meeting notes recorded here:
>> >
>> >
>> >
>> > include "At next meeting MarkM will present a tiny API proposal for maps
>> > and
>> > sets."
>> In today's practice, iterables are ducktyped by the presence of a
>> "length" property.
> IME they're ducktyped by the presence of forEach (this is far from perfect).
> Length has nothing to do with iterability. What about generators?

I don't want to go looking anything up right now, but I have lots of
memories of things looking for the presence of "length" to denote an

>>  I don't think an "[implication of] metric
>> topology" matters to basically anyone who's not a huge language-design
>> nerd. ^_^
> I'm pretty sure math geeks would disagree.

We math geeks are also thin on the ground, so the point of my
assertion (that the number of people who care is vanishingly small)
still holds.

>> Additionally, having multiple names for the "size of" property makes
>> it more difficult to learn, and more difficult to create generic code.
> That's a bold assertion. I'd argue that having two different names for two
> things that are fundamentally different is quite practical. It's especially
> useful for writing generic code ;)

They're not fundamentally different unless you go to some lengths (pun
not intended) to make them so.  "length" is the size of a collection;
it doesn't immediately denote anything about the indexability of said

> But more practically we need different names -- setting length has certain
> expectations that wouldn't hold across all collections. Violating these
> expectations would make things ever more difficult to learn...and generic
> code even harder to write.

This sounds pretty reasonable.  The fact that 'length' is settable on
Array is pretty weird in the first place imo, and as you say, it's
nonsensical for Set/Map.  Okay, I'm more in favor of 'count' or
something being a readonly on all the collections then, as you
suggested in the thread Brendan linked, so that it can become the "new


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