yield syntax (diverging from: How would shallow generators compose with lambda?)

[Neil Mix]

On May 18, 2009, at 12:23 PM, Brendan Eich wrote:

> Making them mandatory is the issue. You can derive whatever comfort  
> you want from 'em, but they are not required and we're not going to  
> start requiring them for return, delete, or typeof. So mandating  
> parentheses for yield is kind of wrong. Obviously the issue would go  
> away if the precedence where the same as delete and typeof. Hence  
> "kind of".

Ah, but delete and typeof are different in that they both *require* an  
argument (terminology be damned, hopefully you understand what I mean  
by that.) And return is different because it can't be used in an  
expression.  See my previous let x = yield-5; example -- let x =  
typeof-5; is unambiguous.

I'm tempted to argue that yield should require an argument, and then  
you could give it precedence equivalent to delete and typeof.  But  
that would make a statement like

   yield x - 1;

utterly confusing.

Set aside for a moment the argument of *which* form of parenthesis for  
a moment, and assume (yield E) wins for now.  Could we redefine yield  
so that it has two forms, a statement form and an expression form?   
The statement form would be
   yield E;
where E is optional.  And the expression form would be
   yield E
where E is required, and yield has precedence equivalent to delete and  
typeof.  Is that possible?  Doing so would render any argument about  
parenthesis moot, since they'd rarely be required, right?