For Loop Desugaring (was: return when desugaring to closures)

Brendan Eich brendan at mozilla.com
Mon Oct 13 22:28:20 PDT 2008


On Oct 13, 2008, at 7:48 PM, Mark S. Miller wrote:

> On Mon, Oct 13, 2008 at 5:00 PM, Jon Zeppieri <jaz at bu.edu> wrote:
>> 1) for (var i = 0; i < len; i++) ... continues to mean what it  
>> means in ES3.
>> 2) for (let i = 0; i < len; i++) ... has the proper scope for 'i'
>> (which you reiterated above), *and* 'i' is rebound -- not mutated --
>> on each iteration.
>> 3) The rewrite rules are the *same,* regardless of whether it's a  
>> "for
>> (var ...)" or a "for (let ...)" loop.
>>
>> At least, that's what I took Mark to mean.  He can correct me if  
>> I'm wrong.
>
>
> You're right. However, the desugaring is more complex than I expected.
> Thanks for asking me to write it down.
>
>  for (<keyword> <varName> = <initExpr>; <testExpr>; <updateExpr>)  
> { <body> }
>
> desugars to (hygienic  renaming aside):
>
> breakTarget: {
>  const loop = lambda(iter = <initExpr>) {
>    <keyword> <varName> = iter;
>    if (! <testExpr>) { break breakTarget; }
>    continueTarget: { <body> }
>    lambda(iter2 = <varName>) {
>      <keyword> <varName> = iter2;
>      <updateExpr>;
>      loop(<varName>);
>    }();
>  };
>  loop();
> }
>
> I believe this meets all your requirements.

Requirement 3 is met because var hoists to the enclosing function.  
This is the part I missed -- well done.

Note that continue; in body translates to break continueTarget; and  
break; translates to break breakTarget; -- you knew that ;-).

> However, in contradiction
> to my original claim, one couldn't usefully say "const" instead of
> "let" with a for(;;) loop.

Why not usefully? It's true the consts do not hoist to leave an effect  
in the variable object, but they do ensure const-ness. The body and  
update cannot mutate the nearest <varName> binding. This is sometimes  
useful and existing const implementations allow for (const...). I do  
not think it should be rejected just yet.

/be


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