From: Hal Fulton <hal9000@...>
Date: 2003-08-30T03:35:17+09:00
Subject: Re: #collect with block modifying receiver

dblack@superlink.net wrote:
>>>I wouldn't like that, personally. I don't remember exactly where, but I
>>>believe that I have a piece of code taking advantage of the fact that a
>>>receiver *can* be modified during iteration. I don't modify during a
>>>#collect, but during an #each. One of the things you can do is:
>>>
>>>  arr.each { |e| arr << f.flatten if f.kind_of?(Array) }
>>
>>Well, if I understand Matz correctly, this behavior is not guaranteed.
>>
>>If that's the case, you should change this to avoid being bitten
>>the way I was.
> 
> 
> I think (Matz - ?) that Matz was talking specifically about changing
> the length of the receiver during an iteration, since that raises the
> question of how many iterations there should be, what the next one
> should be if something gets sliced out, etc.
> 
> Just modifying the objects during an iteration is different, I think.
> #map! does it, for example.  Or:
> 
>   names.each {|name| name.upcase!}
> 
> I'm pretty sure there's no danger with in-place things like this
> (as opposed to things that alter the meaning of the 'place' you're
> 'in' :-)

Yes, I agree. But to avoid confusion for others, let me point out that
Austin's code *does* change the length of the receiver.

However, let's examine this at a slightly finer granularity. It seems
likely to me that it is "safer" to increase the length of a container
(to append) than it is to decrease its length, to add/delete in the
middle, and so on.

The "less safe" behavior was what the randomizing code did. It worked
under 1.6.* and now fails under 1.8.0.

OK, here's a modified question for Matz. If we are iterating over a
list, and the block *appends* to the list, is the behavior guaranteed
and the code thus acceptable?

Hal