# Programming Ruby ### The Pragmatic Programmer's Guide
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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]`

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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/)).

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