Forwards-compatible syntax proposal

[Lars Hansen]

> -----Original Message-----
> From: es4-discuss-bounces at mozilla.org 
> [mailto:es4-discuss-bounces at mozilla.org] On Behalf Of Steven Mascaro
> Sent: 16. mai 2008 22:34
> To: es4-discuss at mozilla.org
> Subject: Forwards-compatible syntax proposal
> 
> Ian Hickson wrote:
> >> Another reaction is that there are ES4 source fragments 
> that /can/ be 
> >> parsed by ES3 compilers, but whose meaning will silently 
> be something 
> >> completely different in the two versions of the language.  
> Here are 
> >> some:
> >> 
> >>   // a generator expression in ES4, a loop calling a function
> >>   // in ES3
> >> 
> >>   function f(x) {
> >>       for ( i=0 ; i < 10 ; i++ )
> >>           yield (f(), i)
> >>   }
> >> 
> >>   // (artificial) three statements in ES3, two in ES4
> >> 
> >>   x = this function 
> >>   f(x)
> >>   {  x++ }
> > 
> > I think this is very dangerous, and unwise, in the context of the
Web. 
> > I strongly believe that if we want to have success in expanding the 
> > Web platform, backwards-compatibility is critical. But that's
another issue.
> 
> I agree. Lars, surely this is unacceptable by your very own reasoning
> --- because there will be ES3 code that will run in ES4, but 
> with silently different behaviour.

I don't know what hixie is getting at exactly (whether he's referring to

the first or the second example).  I've spent some time examining 
compatibility between ES3 and ES4 [1].  Opt-in to ES4 keeps you out of
most trouble.

--lars

[1] http://www.ecmascript.org/es4/spec/incompatibilities.pdf