From: Alan Chen <alan@...>
Date: 2002-11-19T02:42:29+09:00
Subject: Re: Explanations of yield for a newbie?

On Tue, Nov 19, 2002 at 12:52:25AM +0900, christopher.j.meisenzahl@citicorp.com wrote:
> 
> I've been reading the pickaxe book and I think I kind of understand them, but I 
> really need a concise explanation of the point of them and the power of them to 
> put me over the top.
> 
> Any references to more info on this very much appreciated!
> 
> I think one of the points of yield is to allow user defined classes to have 
> easily defined iterators?

Think of a block as an anonymous subroutine. Instead of saying
def func(x,y)
  # ... do stuff
end

it goes

{ |x,y|
  # ... do stuff
}

yield is just the way you call the block passed to the function.
If you have a C/C++ background, it might help to think of blocks as
function pointers and yield as dereferencing a function pointer.
Yield is similiar to a callback function.

Even closer to function pointers in C/C++ is a proc object.

myproc = proc { |x,y| 
  puts "x: #{x}, y #{y}"
}

# You can accept these objects as parameters to functions

def func(x,y,p)
  p.call(x,y)
end

func(1,2,myproc)

Using yield is just a more convenient way to perform the same thing.
The yield version of the above might go as follows:

def func(x,y)
  if( block_given?)
    yield(x,y)
  end
end

func(1,2) { |x,y|
  puts "x: #{x}, y #{y}"
}


-- 
Alan Chen
Digikata Computing
http://digikata.com