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

Brendan Eich brendan at mozilla.com
Mon May 18 12:26:51 PDT 2009

On May 18, 2009, at 12:15 PM, Neil Mix wrote:

> 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.

Yes; I pointed that out earlier in this thread:

"But then almost any algebraic or logical expression computing the  
value to yield would need parentheses, and people would make mistakes  
such as yield a + b where they meant yield(a + b) -- as in Python --  
but got yield(a) + b."

But this objection applies to yield expressions in larger expressions  

> 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?

Not clear. Using yield in a larger expression may nevertheless want to  
yield an expression with precedence lower than unary (delete, !, etc.)  
precedence. ES3 grammar:

UnaryExpression :
      delete UnaryExpression
      void UnaryExpression
      typeof UnaryExpression
      ++ UnaryExpression
      -- UnaryExpression
      + UnaryExpression
      - UnaryExpression
      ~ UnaryExpression
      ! UnaryExpression

Since yield has some some similarity to return I'm still in favor of  
what Python did: low precedence, mandatory parenthesization of the  
whole yield expression in any non-statement context. JS1.7 tried to  
relax this, but it's not worth it as your experience says.


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