From: Joel VanderWerf <vjoel@...>
Date: 2003-07-17T10:08:11+09:00
Subject: Re: callbacks in ruby and using yield in resursion

Paul wrote:
> Hi,
> 
> How do I pass a ruby function as an argument to another ruby function so
> that it can be used as a callback?
> 
> Also, I have a recursive tree traversing function that I would like to
> use yield with. For example:
> 
> def run(parent,level)
   def run(parent, level, &block)
> 	....
> 	yield parent
         block.call(parent)
> 	....
> 	run(child,level+1)#yield????????
         run(child,level+1, &block)
> 	....
> end
> 
> run(root,0){|node|
> 	puts node.get_name
> }
> 
> It will not work because i cant use the yield in the recursive call to
> run.

The first use of the "&block" notation converts the caller-supplied 
block to an instance of Proc and stores it in "block".

The second use of "&block" passes the proc object as the caller-supplied 
block of the recursive call to #run.