(Almost) everything is expression

David Herman dherman at mozilla.com
Fri Nov 11 06:34:24 PST 2011


On Nov 11, 2011, at 3:48 AM, François REMY wrote:

> I think you strongly underestimate the "distinction" problem. ... It's completelty unclear to me. If there's no way to tell what the "return statement" of the block is, there's no way to implement your proposal.

It's actually quite easy to implement Dmitry's proposal, because it's already specified by the ECMAScript semantics! All statements can produce completion values. You can test this yourself: take any statement, quote it as a string, and put it in eval, and you'll get a value (if it doesn't loop infinitely, exit the program, or throw an exception, of course).

As Andreas said, there's a subtler issue of whether there's a simple structure to value-producing substatements, and there's a few problematic cases, but there's already a plan to clean that up at

    http://wiki.ecmascript.org/doku.php?id=harmony:completion_reform

But that's mostly a secondary issue, to keep things more regular and to be more compatible with tail calls.

To answer your specific example:

>   let a = if (foo) {
>      print('a is foo');
>      foo;
>   } else {
>      // do some longer stuff
>   };
> 
> How do you know "foo" is an expression that should be assigned to "a" and that "print('a...')" is not?

Because the then-branch of the if is a block, and the completion value of a block is specified in ECMAScript to be the last completion value produced by its body statements.

Dave



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