for-loops and declaration-like init expressions

[Will Ray]

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
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>>
>
>
>
> --
>     Cheers,
>     --MarkM
>
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