From: Paul Lutus <nospam@...>
Date: 2006-09-11T11:15:23+09:00
Subject: Re: Struggling with Blocks

Hal Fulton wrote:

> Paul Lutus wrote:
>> 
>> "do" reads from a list of items and provides them to its controlling
>> block:
>> 
>> These two forms are interchangeable. One can argue that "do" is an
>> implicit "{ ... }" block, or the reverse. But the point is "do" receives
>> items and operates on them one at a time, then exits when its stream is
>> empty.
> 
> Not strictly true.

Yes, strictly true, IMHO.

> Reemember that the iteration comes from the fact 
> that the each method is invoking the block multiple times.

Not if there is only one item.

array = []

array << "one item"

array.each do |item|
   # one item
end

> 
> Here's a do/end that will only execute once (and by the way
> has no parameters):
> 
>    Dir.chdir(some_dir) do process_something end

In this example, "do" acts on its stream until it is empty. The stream
happens to empty after one item, because that is what the stream contains.

> And by the way, the rescue that can appear between def and end
> is rather an example of what some people want, IIUC. My impression
> is that do/end don't have a rescue basically to avoid unneeded
> complexity and because putting rescue inside {} for orthognality
> looks questionable.

Yep. In any case, one can get the rescue within the do ... end block by
enclosing a begin .. end block. I don't think this will satisfy anyone who
would like to see "rescue" added to do ... end.

I've believed for a long time that a "good" language has a minimum number of
constructs.

-- 
Paul Lutus
http://www.arachnoid.com