On Saturday, September 8, 2012 at 4:01 PM, Allen Wirfs-Brock wrote:
> This is the first of probably several semantic issues WRT generator that will
> need to be resolved in the spec.
>
> Here is the user level issue:
>
> let thrower = {throw () {throw "exception"}};
> function * G(arg = thrower.throw()) {
> //possibly other explicit initialization code which could also throw
> yield arg;
> }
> let g = G(); //do we see "exception" here? (loc 1)
> ... //possibly many lines of code
> g.next(); //or do we see it here (loc 2)
>
If this weren't a generator, we'd see the exception at loc1, correct? Looking
at your example, before I read further, I would assume loc 1
Rick
>
> The generator proposal [1] doesn't really address this issue. It assumes [2]
> that an environment record with the parameter bindings can be created and
> captured at loc 1 without triggering any exceptions and it defers
> instantiation and execution of all of the generator's code body until the
> first "next" call. However, the semantics of parameter default value
> initialization can result in exceptions and more closely links argument
> instantiations with the actual activation of the generator function. You
> can't really build the parameter bindings without performing declaration
> instantiation for the function.
>
> Arguably, exceptions involving initialization of the the parameter would be
> more meaningful at loc 1 (loc 2 could be very remote and in somebody else's
> code). However, the body of the generator may contain other explicit
> initialization code and there really is no way to identify and move
> evaluation of that code (and its possible exceptions) to loc 1. So, even if
> parameter initialization exceptions are made observable at loc 1 there may
> still be other logical initialization-related exceptions that will onlybe
> observable at loc 2.
>
> Which semantics do we want? Deferring parameter initialization to loc 2 may
> be somewhat simpler from a specification perspective. It's hard to say which
> would have bigger implementation impact.
>
> Allen
>
> [1]: http://wiki.ecmascript.org/doku.php?id=harmony:generators
> [2]: http://wiki.ecmascript.org/doku.php?id=harmony:generators#calling
>
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