From: Joel VanderWerf <vjoel@...>
Date: 2002-01-01T03:27:20+09:00
Subject: [ruby-talk:29879] Re: Proc.class vs yield

David Alan Black wrote:
> 
> Hi --
> 
> On Tue, 1 Jan 2002, Joel VanderWerf wrote:
> 
> > David Alan Black wrote:
> >
> > > Hmmm... I wonder whether part of what you're not liking is the fact
> > > that the {} operator is overloaded: it's a block constructor and also
> > > a hash constructor.  So there needs to be a way to deal with things
> > > like:
> > >
> > >   some_method( { "hi" }, { "one" => 1 }, { |x| puts x })
> >
> > There is still some room to overload {} a bit more. The syntax
> >
> >   some_method {|x| x+1} {|y| y-1}
> >
> > has no meaning currently--it's a parse error. Could this syntax be given
> > a meaning so that
> >
> >   def some_method(&b1, &b2)
> >   end
> >
> > allowed the method to use both blocks?
> 
> What would yield do?

Since you're explicitly converting each block to a proc, you don't need
yield in this case. Maybe yield would look for a third, unobjectified,
block?

> > I'm not sure I'd propose this for RCR, since I haven't seen a use for it
> > that couldn't be handled some other way. But still it would be kinda
> > nice. It would allow you to write control structures a la Tcl:
> 
> That would be a mighty big RC :-)
> 
> >   while {<test>} {
> >     <do stuff>
> >   }
> >
> 
> I don't know Tcl.  What does that structure give you?

In Tcl, { ... } denotes a string without interpolation, like '...' in
Ruby. So if you want to define a C-style for-loop in Ruby, you could,
but it would look like:

  for {$x = 0} {$x < 100} {$x += 1} {
    puts $x
  }

One problem: to use a local variable, you'd still have to reference it
above the for-loop, or else the references in each of the blocks would
be unrelated.

Anyway, I still don't see a desperate need for this. When I really need
to pass several blocks, I just use the auxilliary object trick with
instance_eval. That has the advantage of associating a name with each
block, so you don't have to remember the order.

--
Joel VanderWerf                          California PATH, UC Berkeley
mailto:vjoel@path.berkeley.edu                     Ph. (510) 231-9446
http://www.path.berkeley.edu                       FAX (510) 231-9512