(Almost) everything is expression

Brendan Eich brendan at mozilla.com
Fri Nov 11 08:36:49 PST 2011


On Nov 10, 2011, at 11:07 PM, Dmitry Soshnikov wrote:

> Brendan and Dave mention explicit semicolon. Yes, it's seems so by the grammar (though, have to check more precisely), but it can be acceptable price.

No, it is  a runtime incompatibility that shifts meaning, without errors.

switch (x) { case 1: (function (){return 42}); break; default: (function (){return 99}); }
(a[i].b()).c(d)

The switch is now the callee expression in a call taking one actual parameter, a[i].b().

The same can happen with leading [, unary +/-, and / as regexp delimiter -- any lexeme that can both start a statement and continue an expression.


> P.S:
> 
> Regarding Dave's `do { .. }` -- we may omit `do` and just evaluate the block.
> 
> let a = {
>  print('doing stuff');
>  100;
> };
> 
> It's of course seems ambiguous with an object initialiser (at first glance), but it's only at first glance. Obviously there is a code inside to evaluate.

I worked on this, based on ideas from Breton Slivka and Doug Crockford. Please see

http://wiki.ecmascript.org/doku.php?id=strawman:arrow_function_syntax

and

https://mail.mozilla.org/pipermail/es-discuss/2011-June/015568.html

This is not going to fly in a grammar that we validate using LR(1) parsing.

Block-lambdas require {|| at least to defer evaluation until invocation, whereas any block-expression would be immediately evaluated. This could be a point of confusion.

Altogether, this says Dave's 'do' proposal is better because EIBTI.

/be


> 
> Cheers,
> Dmitry.
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