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Class: Continuation

Relationships & Source Files
Inherits: Object
Defined in: cont.c,
cont.c

Overview

Continuation objects are generated by Kernel.callcc, after having required continuation. 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:

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

produces:

Freddie
Herbie
Ron
Max

Also you can call callcc in other methods:

require "continuation"

def g
  arr = [ "Freddie", "Herbie", "Ron", "Max", "Ringo" ]
  cc = callcc { |cc| cc }
  puts arr.shift
  return cc, arr.size
end

def f
  c, size = g
  c.call(c) if size > 1
end

f

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

require "continuation"
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
}
puts

produces:

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

Instance Method Summary

Instance Method Details

#call(args, ...) #[](args, ...)
Also known as: #call

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]
[ GitHub ]

  
# File 'cont.c', line 1271

static VALUE
rb_cont_call(int argc, VALUE *argv, VALUE contval)
{
    rb_context_t *cont = cont_ptr(contval);
    rb_thread_t *th = GET_THREAD();

    if (cont_thread_value(cont) != th->self) {
	rb_raise(rb_eRuntimeError, "continuation called across threads");
    }
    if (cont->saved_ec.protect_tag != th->ec->protect_tag) {
	rb_raise(rb_eRuntimeError, "continuation called across stack rewinding barrier");
    }
    if (cont->saved_ec.fiber_ptr) {
	if (th->ec->fiber_ptr != cont->saved_ec.fiber_ptr) {
	    rb_raise(rb_eRuntimeError, "continuation called across fiber");
	}
    }
    rollback_ensure_stack(contval, th->ec->ensure_list, cont->ensure_array);

    cont->argc = argc;
    cont->value = make_passing_arg(argc, argv);

    cont_restore_0(cont, &contval);
    return Qnil; /* unreachable */
}

#call(args, ...) #[](args, ...)

Alias for #[].