[ruby-talk:7167] Re: |<variable>| notation?

Mitch Vincent <[e-mail]> 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