From: Dan Debertin <airboss@...>
Date: 2003-08-06T23:02:33+09:00
Subject: Re: Why does Ruby have callcc?

Jim Bob <invalid@invalid.com> writes:

> So, how'd it get there? Is there some killer use for callcc that I'm
> missing? Was it just for the "gee whiz" factor?

I can't explain the "how'd it get there" part; I'll leave that to Matz
and friends.

Continuations are great for any situation in which you not only need
to jump out of a context (multiple nested loops, etc.), but also get
back in to the same context you left. You can't do that with
catch/throw. 

A good example of this is co-routines -- you need to save your state
in the first function, start the second one, then jump back to where
you were in the first, and so on. This example runs two iterators
simultaneously:

# Iterate simultaneously from a to b, and from b to a
def updown(a, b)
  iter1, iter2 = ( a < b ? [a.method(:upto), b.method(:downto)] :
                   [a.method(:downto), b.method(:upto)] )
  f, cc1, cc2 = nil

  iter1.call(b) do |o|
    callcc do |cc1|
      f = o
      cc2.call if(cc2)
      iter2.call(a) do |i|
        callcc do |cc2|
          yield f, i
          cc1.call if(cc1)
        end
      end
    end
  end
end

Of course, one can't discount the considerable "Gee wiz" factor of
continuations ;-).

Dan
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