Generators (in similar manner and for similar reasons as in PEP-380) can
return a value via a `return` statement. This requires a pair {value,
done}, you cannot do it with a sentinel such as your item (3).
BTW at ECOOP in Prague this past week, I heard from folks (I forget
whether it was Till Schneiderheit or Andy Wingo) that the
object-per-iteration cost avoidance optimizations are about to land in
one of the top engines.
/be
David Bonnet wrote:
Sorry for bringing this up again, especially now that the specification has
been finalised. However, I can’t really understand the rationale behind the
design of `Iterator.next()`.
There are at least three possible options for the behaviour of
`Iterator.next()`:
1. Returning a value and throwing `StopIteration` when done.
2. Returning {value, done} object with `done = true` when done.
3. Returning a value and returning `Symbol.iterationDone` when done.
I understand that option 1 would require trapping the exception.
Option 2 would require creating an object at each call, which as discussed in
this thread, would require non-trivial optimisations.
Option 3, however, seems IMHO to have no caveats, and would require adding
another symbol (`Symbol.iterationDone`) along with `Symbol.iterator`.
So why was option 2 chosen? Why not option 3? All I could find is a discussion
that still refers option 1:
http://wiki.ecmascript.org/doku.php?id=harmony:iterators
Cheers,
David
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