Your example would NOT see the new elements because
SequenceableCollection>>do: is implemented as:

do: aBlock
 "Refer to the comment in Collection|do:."
 1 to: self size do:
  [:index | aBlock value: (self at: index)]

See, end of the array was computed before the loop started.

If you shortened the array, you would have an error pop up.

"become:" affecting an active iteration is wrong.  Iterations can only be
clean if they're over a snapshot not over a live collection anyway.  Ok, we
can't afford pure cleanness but there's no point in pushing the language
toward an idiom that can't even be defined without contradictions.

What if "become:" changes the type of array to a different type?

You could argue that an array of a different size IS a different type.  It
is in many languages.

No, THERE IS NOTHING WRONG locking down the array and iterating over it.
If the code calls "become:" then continuting the iteration over the old
array is no worse than trying to continue a loop over the new (I would say
"wrong") object.

In fact, finishing the iteration with the object you started with _is_
_cleaner_.

If you don't agree with me on that, then maybe you'll decide that _it_
_doesn't_ matter_ _because_ _code_ _shouldn't_ _call_ _become_ _inside_ _of_
_and_ _iterator_ _on_ _that_ _object_.

Whew!

Joshua Scholar
----- Original Message ----- 
From: "Tim Rowledge" <tim at sumeru.stanford.edu>
To: <jscholar at access4less.net>; <squeak-dev at lists.squeakfoundation.org>
Sent: Wednesday, December 01, 2004 11:46 AM
Subject: Re: Adding loop primitives/optimizations (was Making
Set/Dictionaryetc. loops more robust)


> "Joshua Scholar" <jscholar at access4less.net> wrote:
>
> > Since I was only talking about iterating over primitive types like
arrays,
> > it's not possible to change the size of the collection while iterating
over
> > it.
> Really? Consider what happens in this:-
>
> |array newArray|
> array := Array new:10.
> 1 to: array size do:[:i| array at: i put: i squared]. "ok"
> array do:[:el|
> newArray := Array new: el +3.
> array become: newArray]
>
> Warning - this code might cause brain damage. Never underestimate the
> excitment #become: can cause, nor how often some demented ninny will
> spring it upon you.
>
> A more prosaic example might be a stream on an Array. You can add extra
> elements to the array which in the Squeak implementation replaces the
> array with a bigger one BUT IIRC older Smalltalks used a become to do
> that grow. Now add in multiple threads and life can get very
> complicated.
>
> tim
> --
> Tim Rowledge, tim at sumeru.stanford.edu, http://sumeru.stanford.edu/tim
> Useful random insult:- Talks to plants on their own level.