From: Anders Janmyr <anders.janmyr@...>
Date: 2005-11-24T01:11:26+09:00
Subject: Continuations

Hello Everyone,
I am trying to come to terms with continuations and I am failing  
miserably.

I have a method "traverse" which basically is a traversal algorithm  
for a sudoko game.
To learn more about continuations I tried to write one that used  
continuations instead: traverse_cc.
This does not work at all. I have tried many different usages but the  
main problem seems to be that the local variable state is not  
restored when the continuation is called. I thought that it was  
supposed to do that.

Any help will be appreciated

Regards
Anders

   def traverse(debug)
     @solve_count += 1
     print "F" if debug
     return true if @available_positions.empty?
     position = @available_positions.pop
     position.reset
     position.possible_numbers.each do |num|
       position.place(num)
       return true if traverse(debug)
     end
     position.place(nil)
     @available_positions.push position
     print "B" if debug
     return false
   end



   def traverse_cc(debug)
     $c_stack = []
     available_positions = @available_positions.dup
     position = nil
     possible_numbers = nil
     until (available_positions.empty?)
       @solve_count += 1
       callcc do |cont|
         $c_stack.push cont
         print "F" if debug
         position = available_positions.pop
         position.reset
       end
       possible_numbers = position.possible_numbers

       if possible_numbers.empty?
         print "pop #{available_positions.size}:# 
{possible_numbers.size}:#{$c_stack.size}\n"
         $c_stack.pop.call
       else
         num = possible_numbers.pop
         position.place(num)
       end

       print "#{available_positions.size}:#{possible_numbers.size}:# 
{$c_stack.size}, "
     end
     return true
   end