From: Paul Lutus <nospam@...>
Date: 2006-09-11T14:10:24+09:00
Subject: Re: Struggling with Blocks

Ken Bloom wrote:

/ ...

> Let me try a direction in answering this question. I don't know anything
> about the internals of Ruby to know whether this is right or not, but
> perhaps someone can confirm.
> 
> do..end and {...} blocks are lexical closures that remember their
> surrounding binding. This requires setup and teardown code for dealing
> with bindings that could otherwise go out of scope.
> 
> begin...end can't be passed around like do...end, and so it doesn't need
> the same kind of setup and teardown code for dealing with bindings.
> 
> Different syntax is needed because the ruby interpreter isn't smart enough
> to know which situation is which.
> 
> Is this close to correct?

It's reasonable. I can put a "do ... end" block in a Proc object (because
that's what it is) but I can't do this with begin ... end (unless the
begin ... end block is nested in a do ... end block). So they differ that
way. This may not be a particularly persuasive argument for or against
rescue within do ... end, because I can put a rescue clause in a begin ...
end block that is nested in a do ... end block.

Like so:

-------------------------------------------------

#! /usr/bin/ruby

p = Proc.new { |s|
   begin
      puts s
      y = x/0
   rescue
      puts "Error!"
   end
}

[ 'a' , 'b', 'c' ].each { |s| p.call(s) }

Output:

a
Error!
b
Error!
c
Error!

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