Class: Continuation (Ruby 2.3.4)

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    • cont.c

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    Continuation

    Continuation objects are generated by Kernel#callcc, after having +require+d 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

    Public Instance Methods

    cont[args, ...] click to toggle source

    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]
    
     
                   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, ...) click to toggle source

    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]
    
     
                   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 */
    }