External Iteration Using Continuations

Occasionally you see discussions (like on the IwannaLearnRuby page) about InternalIteration vs. ExternalIteration. I think those discussions are evidence of a LanguageSmell. In a language that supports CallWithCurrentContinuation, you don't need to choose; you can build a general-purpose ExternalIterator.

For example, in the RubyLanguage:

 # A class that transforms an InternalIterator
 # (like :each or :each_index) into an object
 # that you can call :next on to keep getting
 # successive values.
 class ExternalIterator
   def initialize(iterationMethod)
     @iterationMethod = iterationMethod
     startLooping
   end

   def startLooping
     # Don't do anything yet - just save the current
     # context and return.
     callcc { |@context_of_looping_method|
       return
     }

     # If we got here, it's because someone called the
     # "next" method. Start running the iteration method,
     # but after each iteration, save the current context
     # and jump back to the context of the "next" method.
     @iterationMethod.call { |nextValue|
       callcc { |@context_of_looping_method|
         @context_of_next_method.call(nextValue)
       }
     }

     doneLooping!
   end

   def doneLooping!
     @context_of_looping_method = nil
     @context_of_next_method.call
   end

   def doneLooping?
     @context_of_looping_method.nil?
   end

   def next
     if not doneLooping?
       # Jump back to the looping method. (The
       # looping method will then go one more
       # time through the loop and then jump
       # back here.)
       callcc { |@context_of_next_method|
         @context_of_looping_method.call
       }
     end
   end
 end

 class Object
   # Just a convenience method.
   def iterator(methodName)
     ExternalIterator.new(method(methodName))
   end
 end

 # Some simple tests.
 if __FILE__ == $0
   def fibonacci
     yield 1
     a, b = 1, 1
     loop {
       yield b
       a, b = b, a+b
     }
   end

   f = iterator(:fibonacci)

   puts "Printing the first six Fibonacci numbers..."
   puts f.next   # => 1
   puts f.next   # => 1
   puts f.next   # => 2
   puts f.next   # => 3
   puts f.next   # => 5
   puts f.next   # => 8

   puts "Printing the indices of a four-element array..."
   g = ['a', 'b', 'c', 'd'].iterator(:each_index)
   puts g.next   # => 0
   puts g.next   # => 1
   puts g.next   # => 2
   puts g.next   # => 3
   puts "And here's what happens if we keep going off the end of the array..."
   puts g.next   # => nil
   puts g.next   # => nil
   puts g.next   # => nil
 end

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CategoryContinuation