From: Dave Cantrell <ruby-talk@...>
Date: 2006-02-08T13:57:55+09:00
Subject: Re: What's the difference between copying and sharing in closure?

> Contrast this to Ruby:
> 
> 	def do_yield()
> 		yield
> 	end
> 
> 	foo = "FOO"
> 	do_yield() {
> 		foo = "BAR"
> 	}
> 	puts foo # Outputs "BAR".
> 
> Which means you can assign to local variables in the closure's lexical scope 
> from inside the closure.

Question: How is the above different from, say in pseudo code:

   function do_something(in)
     foo = in
   end function

   foo = "FOO"
   do_something("BAR")

   print foo  # should still output "BAR", right?

Other than the fact that you can create the block logic at call time 
rather than at declaration time, what is the difference in how the two 
scopes are handled? In both the Ruby example and the pseudo-code case, 
the variable foo is a globally-scoped variable (well, for that code 
snippet anyway) and therefore should be accessible to any function that 
is subordinate to it.

After reading the example above I was led to try something similar. I'm 
definitely a Ruby Nuby (I had to research the IO routines to pad the 
code for this e-mail) so I had to fiddle with the variable scoping for a 
while to get it to work. It wasn't until I managed to get the variable 
assignment into the right place that I got something other than nil or a 
proc object.

But in the end, writing this helped me better understand the way Ruby 
shares variables in closures. Though I'm still confused as to how it's 
truly different from regular scoping rules.

   class MethodList
     def initialize
       @methods = []
     end

     def add(&block)
       @methods << block
     end

     def run(param)
       i = param
       @methods.each { |m| i = m.call(i) }
       i
     end
   end

   m = MethodList.new
   m.add { |x| x + 1 }
   m.add { |x| x + 1 }
   m.add { |x| x + 1 }

   puts m.run(1)


Which, of course, outputs 4. What tripped me up was getting the hang of 
the proper way to update the i variable and the proper way to output the 
result. I first tried to have the puts in each of the m.add {} blocks -- 
then I realized I'm overdue for sleep. Doh. Can you see me getting the 
hang of blocks? :)

Any clarification greatly appreciated.

Thanks,
-dave