From: Mitch Vincent <mvincent@...>
Date: 2000-12-14T03:37:28+09:00
Subject: [ruby-talk:7168] Re: |<variable>| notation?


Excellent. Thanks very much... 

Oh and the book is on it's way as we speak :-)
 
-Mitch

On Thu, 14 Dec 2000, Dave Thomas wrote:

> Mitch Vincent <mvincent@venux.net> writes:
> 
> > Thread.start(@listen.accept) do |aConnection|
> > 
> > Ok, I'll take a guess here but I want to make sure i'm right and that I
> > understand this..
> > 
> > Does the above assign the returning value of @listen.accept to the
> > variable aConnection (Which is more or less a socket descriptor)? I
> > suppose that's pretty obvious, as that's how it does seem to work in the
> > rest of the code, I've just never seen an assignment made like that.. 
> 
> And a good guess it is. That's exactly the effect, but perhaps not
> quite for the reason you might expect.
> 
> The |...| notation is used with blocks. A block in Ruby is a chunk of
> code that is remembered, not executed, at the point of definition. In
> a way, a block is link an anonymous subroutine, except a block has
> some additional magic we don't need to think about here ;-)
> 
> If a block is like a subroutine, then the stuff between the ||'s are
> the block's parameters. So, in the example above, 'aConnection' is a
> parameter for the block that follows the Thread.start.
> 
> So, how you you call one of these blocks? It's pretty
> simple. A block can only be defined by associating it with a method
> invocation. Within that method, there's a special statement, 'yield',
> which calls back out to the block. You can pass parameters to yield,
> which are then assigned to the block's formal parameters.
> 
> So, Thread.start starts the corresponding block as a new thread, and
> passes any parameters given to Thread.start on to the block
> itself. Why does it go through this level of indirection?
> 
> You'll have to buy the book ;-)
> 
> 
> 
> Dave
>