On Jul 30, 2010, at 3:02 PM, Oliver Hunt wrote:
> On Jul 30, 2010, at 2:56 PM, Brendan Eich wrote:
>
>> To avoid this confusion you can add new syntax (|for each| or whatever,
>> doesn't matter). I've argued in recent posts that it is better from a global
>> and long-term point of view to reform for-in after Python, than to condemn
>> it and grow the language with new and generally more verbose, yet similar,
>> syntax.
>
> It annoys me that this doesn't leave a convenient way to iterate arrays, for
> (... in someArray) will forever do the braindead thing.
Me too, and I've seen people (myself included) make the mistake of writing
for (i in [0,1,1,2,3,5,8,13]) ...
wanting the values not the keys.
> If we look to pythonic behaviour we see that python's for..in syntax has
> always iterated arrays by value rather than index.
Indeed.
> By overloading for(in) we are effectively saying that there will never be a
> simple way to iterate arrays by value directly, because no one can even
> extend the builtin array type be have a generator for iteration because doing
> so would be too fragile.
This "never be a simple way" is not true in JS1.7+:
js> Array.prototype.__iterator__ = function () { for (let i = 0; i <
this.length; i++) yield this[i]; };
(function () {for (let i = 0; i < this.length; i++) {yield this[i];}})
js> for (v in [3,4,5]) print(v)
3
4
5
The unstratified, ugly-named __iterator__ meta-method is the getter or factory
for finding or creating an appropriate iterator. I use a generator, since it is
the simplest way of writing such a factory.
With Harmony we stratify such meta-programming using proxies, so you can't use
the prototype chain to find the iterate handler-trap -- you must directly
reference a Proxy instance after 'in' in the for-in head.
Could we "fix" Harmony to iterate values not keys for Arrays, by fiat, and say
"opt-in versioning, pay this migration tax, it is worth it"? I don't think so,
but we can discuss.
Perhaps there's a modular way to allow one-time, explicit opting into a mode
where for-in iterates array values, not keys. The obvious criticism is modes
are bad, don't add more knobs to the language.
/be
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