On Feb 2, 2012, at 5:07 PM, Jason Orendorff wrote:
> On Thu, Feb 2, 2012 at 5:52 PM, Allen Wirfs-Brock <[email protected]>
> wrote:
>> Unfortunately, having thought about it a bit since the meeting, I'm less
>> convinced. We didn't look too deeply at possible capture scenarios such as:
>>
>> let geti;
>> for (let i=(geti = function() {return i},expr), expr, incr = function(i++),
>> decr=function(i--), cmp=function(){return i<n}; cmp();incr()) {
>> let j=i;
>> i +=10;
>> decr();
>> ...
>> If (j+-9 !== i) ...
>> ...
>> }
>
> What?! We should not reward people writing code like that.
>
> I would be fine with the initializer-expression not being in the scope
> of i at all.
No reward, it is simply what the current syntax permits, and it is important
that semantics are consistently applied. The initialization semantics I assume
here are exactly the same that would apply if if the let clause of the for was
pulled out and used as a standalone statement. In general, excluding i from the
visibility of the initializer expression would prevent writing recursive
functions definitions such as:
const fact = function (n) {return n<=1 ? 1 : n*fact(n-1)};
for-in works well with a per iteration binding because there is no header code
that runs between iterations. The various desugarings that have been proposed
to provide a fresh copy for each iteration of this form of for work for
propagating simple values between the header and each iteration but not for
closure captures.
Plus, the desugarings aren't things that are really suitable for teaching the
semantics to everyday JS programmers. You instead have to say something like:
Ok, this is really complicated but here goes. For each let/const declared in
the for header, a fresh variable is located in the loop body for each iteration
of the loop. However, the values of the loop variables are automatically copied
from the previous iteration into the next iterations. This means that basic
expression operator will work in the loop header pretty much like you would
expect. But be careful if you use any function expressions in the for header
because the loop variables they reference may not be from the current iteration
and any changes to loop variable they make may not have the effect you
intended. But, hey you probably shouldn't do those things so it really doesn't
matter what it really does.
Allen
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