Programming Ruby

The Pragmatic Programmer's Guide

Previous < Contents ^
Next >
class Continuation
Parent: Object
Version: 1.6

Index:

call


Continuation objects are generated by Kernel#callcc . They hold a return address and execution context, allowing a nonlocal return to the end of the callcc block from anywhere within a program. Continuations are somewhat analogous to a structured version of C's setjmp/longjmp (although they contain more state, so you might consider them closer to threads).

For instance:

arr = [ "Freddie", "Herbie", "Ron", "Max", "Ringo" ]
callcc{|$cc|}
puts(message = arr.shift)
$cc.call unless message =~ /Max/
produces:
Freddie
Herbie
Ron
Max

This (somewhat contrived) example allows the inner loop to abandon processing early:

callcc {|cont|
  for i in 0..4
    print "\n#{i}: "
    for j in i*5...(i+1)*5
      cont.call() if j == 17
      printf "%3d", j
    end
  end
}
print "\n"
produces:

0:   0  1  2  3  4 1:   5  6  7  8  9 2:  10 11 12 13 14 3:  15 16

instance methods
call cont.call( [ args ]* )

Invokes the continuation. The program continues from the end of the callcc block. If no arguments are given, the original callcc returns nil. If one argument is given, callcc returns it. Otherwise, an array containing args is returned.

callcc {|cont|  cont.call } nil
callcc {|cont|  cont.call 1 } 1
callcc {|cont|  cont.call 1, 2, 3 } [1, 2, 3]


Previous < Contents ^
Next >

Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide"
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)).

Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.