From: James Edward Gray II <james@...>
Date: 2007-11-14T08:35:32+09:00
Subject: Re: recursion with blocks

On Nov 13, 2007, at 5:12 PM, Mike Perham wrote:

> Sort of.

We'll get there…

> I'm still unclear how the actual node is propagated back up as
> the result of the find call due to the name == "right" block returning
> true.

We're calling a block recursively, just as we would a method.  The  
typical trick for getting a result from that is just to hand a return  
value back up the call stack.  For example:

#!/usr/bin/env ruby -wKU

class Named
   def initialize(name, child = nil)
     @name  = name
     @child = child
   end

   def find_by_name(&block)
     if block[@name]
       self
     elsif @child
       @child.find_by_name(&block)
     end
   end

   def to_s
     @name
   end
end

names = %w[one two three].reverse.inject(nil) do |child, name|
   Named.new(name, child)
end

puts names.find_by_name { |n| n[0] == ?t     }
puts names.find_by_name { |n| n    == "four" }

__END__

Does that make more sense?

> Now how do I put this on a class which is an ActiveRecord (i.e. the  
> find  method is already taken)?

find() has an alias called detect(), which I don't believe  
ActiveRecord hides with a method of its own.

> BTW Saw your talk at the LSRC.  I was very inspired to use Ruby for
> everything gluey.  :-)

Glad to hear it!

James Edward Gray II