From: Florian Gross <flgr@...>
Date: 2003-12-20T10:20:46+09:00
Subject: Re: Could I have an example of using Continuations as 'co-routines'?

Asfand Yar Qazi wrote:

> Hi,

Moin!

> Here's my problem: instead of using setjmp/longjmp to script my game's 
> story-line (scripting in C++, tut-tut, what was I thinking?!), I 
> obviously want to use a Ruby feature, and I assume Continuations are the 
> thing to do it.

> Anyway, could I have a simple example to start with?

I did this short snipped which implements a simpler interface for 
Continuations to stop them from causing so much headaches for me:

def Continuation.create(*args, &block)
   args = [args] if not args.nil? and not args.is_a? Array

   cc = nil
   result = callcc {|c| cc = c; block.call(cc) if block and args.empty?}

   result ||= args
   return *[cc, *result]
end

Here's a simple example of a program counting from 1 to 5 using 
Continuations:

continuation, number = Continuation.create(1)
if number <= 5
   puts number
   continuation.call(number + 1)
end

And here's one showing how Continuations could be used for coroutines. 
However, as you may have noticed, this is quite clumsy because lots of 
repetitive code passages -- even if they can be factored out into 
something reusable I would probably still be uncertain about using 
Continuations for Coroutines:

def give_numbers
   @continuation.call(true) if @continuation

   @continuation, done = Continuation.create(false)
   return 1 unless done

   @continuation, done = Continuation.create(false)
   return 2 unless done

   @continuation, done = Continuation.create(false)
   return 3 unless done

   @continuation = nil
   return 4
end

give_numbers # => 1
give_numbers # => 2
give_numbers # => 3
give_numbers # => 4
give_numbers # => 1

> Thanks,
>     Asfand Yar

Regards,
Florian Gross