From: "Florian Groß" <flgr@...>
Date: 2003-08-09T05:14:00+09:00
Subject: Re: Why does Ruby have callcc?

Moin!

Ben Giddings wrote:

> Do you have any suggestions on how to improve my Continuation add-ons, 
> or are you happy with just using callcc?

I tried to make a simpler replacement to callcc and I ended up with this:

   def Continuation.create(*args, &block)
     cc = nil; result = callcc {|c| cc = c; nil}
     result ||= args
     return *[cc, *result]
   end

I didn't want to call this Continuation.new because it can return 
multiple values.

Though the following simple examples are working well I would be 
interested to know if the simpler syntax comes at the loss of 
functionality.

1) Word swapper:
   cc, first, second, called = Continuation.create("Hello", "World")
   puts first + " " + second
   cc.call(second, first, true) unless called
   ^D
   Hello World
   World Hello

2) Counter:
   cc, counter = Continuation.create(10)
   print counter, " "
   cc.call(counter - 1) if counter > 0
   puts
   ^D
   10 9 8 7 6 5 4 3 2 1 0

3) Inject:
   module Enumerable
     def new_inject(initial=nil)
       array = self.dup
       initial = array.shift if initial.nil?
       cc, state, array = Continuation.create(initial, array)
       state = yield(state, array.shift)
       cc.call(state, array) if array.length > 0
       return state
     end
   end
   puts [1, 2, 3].new_inject {|state, item| state + item}
   ^D
   6

Regards,
Florian Gro�