for-loops and declaration-like init expressions

[Mark S. Miller]

Consider the function and class declarations in these positions to be
equivalent to:

    for (let x = function(){}; ...

and

    for (let x = class(){}; ...

respectively.




On Thu, Jun 5, 2014 at 8:32 AM, Will Ray <wray167 at gmail.com> wrote:

> Mark, could you explain how the condition and iteration statements in the
> for loop interact with such an initialization? I agree that banning these
> seems like the clearer way to go.
>
>
> On Thu, Jun 5, 2014 at 10:17 AM, Mark S. Miller <erights at google.com>
> wrote:
>
>> Why not accept these as for-loop initializations, so that x actually has
>> that function and that class as its initial value in the first iteration of
>> the loop? To me, that's the least surprise. Since this is a position in
>> which some declarations are accepted, anything there that looks like a
>> declaration should be a declaration.
>>
>> However, I agree that banning these is much less surprising than allowing
>> them as expressions rather than declarations.
>>
>>
>>
>> On Thu, Jun 5, 2014 at 7:58 AM, Andreas Rossberg <rossberg at google.com>
>> wrote:
>>
>>> C-style for-loops allow declarations as init statements, but only some
>>> of them. Yet, the others (function and class) are actually
>>> syntactically legal in that position as well, because they are simply
>>> parsed as expressions. Consider:
>>>
>>>   let x = 0
>>>   for (let x = 1; ;) x  // 1
>>>   for (const x = 1; ;) x  // 1
>>>   for (function x(){}; ;) x  // 0
>>>   for (class x(){}; ;) x  // 0
>>>
>>> I think these latter two examples violate the principle of least
>>> surprise. I wonder if it wouldn't be cleaner to rule them out, by
>>> imposing the same lookahead restrictions on for-loop init expressions
>>> as there are for expression statements.
>>>
>>> The one caveat is that for function, that would actually be a breaking
>>> change, but is it likely to be a real world one?
>>>
>>> What do you think?
>>>
>>> /Andreas
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>
>>
>>
>> --
>>     Cheers,
>>     --MarkM
>>
>> _______________________________________________
>> es-discuss mailing list
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>> https://mail.mozilla.org/listinfo/es-discuss
>>
>>
>


-- 
    Cheers,
    --MarkM
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