Generator issue: exceptions while initializing arguments

Brendan Eich brendan at
Sat Sep 8 15:51:29 PDT 2012

Allen Wirfs-Brock wrote:
> On Sep 8, 2012, at 3:20 PM, Brendan Eich wrote:
>> SpiderMonkey (Firefox 15 and newer has default parameters):
>> js>  function f(a = g) { function g(){}; return a; }
>> js>  f()
>> function g(){}
>> So function g is hoisted and a defaults to it, as expected.
> While I agree that the above is reasonable behavior.  It wasn't the consensus that was reach earlier this year at the Jan.  (or may March??) meeting.  What we agreed upon is that default value expressions in parameter lists have visibility to the left and upward in scope but do not have visibility of anything declared within the curlies that surround the body.   So, in the above example, g should be a reference error when evaluated as a default value initializer.
> As the NOTE in step 9 of 10-5-3 says:
> 	NOTE	Binding Initialisation for formals is performed prior to instantiating any non-parameter declarations in order to ensure that any such local declarations are not visible to any parameter Initialisation code that may be evaluated.

You're right, I had forgotten that.

Is it well-motivated other than in the naive left-to-right sense, which 
function hoisting already violates? Perhaps, because parameters to the 
right are not yet bound.

If so, then I can live with it, and g in the f(a = g) bit above would 
use an outer g, if any (or throw on undefined g).

But this still does not mean the implicit |yield;| at entry to generator 
function should be other than observably "after" parameter defaulting, 
so that we get the throw at loc 1. Right?

IOW, this is a tangent, good to nail down (sorry for prying it up), but 
not a crucial experiment for generator default parameter semantics.


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