Class: Continuation

Inherits:

Object

• Object
• Continuation

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

• #[](*args) ⇒ Object

  Invokes the continuation.

• #call(*args) ⇒ Object

  Invokes the continuation.

Instance Method Details

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

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]

# File 'cont.c', line 1068

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

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

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

    /* restore `tracing' context. see [Feature #4347] */
    th->trace_arg = cont->saved_thread.trace_arg;

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

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

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]

# File 'cont.c', line 1068

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

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

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

    /* restore `tracing' context. see [Feature #4347] */
    th->trace_arg = cont->saved_thread.trace_arg;

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