On 2010-11-23, at 14:14, Brendan Eich wrote:
> On Nov 23, 2010, at 5:19 AM, P T Withington wrote:
>
>> On 2010-11-22, at 02:37, David Herman wrote:
>>
>>> if we allowed for-in to be overloaded, I would tell people that they should
>>> deprecate the legacy for-in and replace it with an explicit iterator such
>>> as:
>>>
>>> for (x in keys(obj))
>>
>> I have learned a mnemonic for for-in: that it is iterating using the `in`
>> operator. You propose that I unlearn that? Or in your new hypothetical
>> world does the `in` operator also get overloaded?
>
> Excellent question. One (Java extension language) answer:
>
> http://www.cs.cornell.edu/Projects/jmatch/
>
> Python allows unstratified meta-programming of both its for-in
> loop/comprehension syntax *and* its |in| operator.
>
> Harmony Proxies allow meta-programming of |in| already, via the |has| trap.
> So the answer to your quesiton "does the `in` operator also get overloaded?"
> is "Yes, but you have to write two traps, iterate and has".
How does the `in` in for-in decide which of it's overloaded meanings applies?
Based on the type of the operand? Based on the existence of the enumerate or
iterate trap on the operand?
And despite the `in` in for-in either enumerating or iterating, the `in`
operator only has a single associated trap. The non-parallelism is bugging me.
>> Ramdom thought: Can I use destructuring in for-in?
>>
>> for ({key:value} in enumerable)
>>
>> for ([value] in iterable)
>
> Absolutely. Destructuring (separate proposal but composes well) applies to
> all LHS and binding forms.
I was being too subtle. I was suggesting something like your JS 1.7 example,
where the 'top-level' destructuring is a pattern for the `in` operation.
`{key: value}` means I want the property keys and their values, `[value]` means
I want the values, and `key` is (backward-compatible) shorthand for `{key: _}`.
Destructuring iteration over an hash of triples:
for ({key: [s, v, o]} in tripledb) ...
Too cute?
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