Re: (Perl6) Groking Continuations

in reply to (Perl6) Groking Continuations

Well, I can't claim to great knowledge on this, but I'll give it a go. Schemers out there will probably laugh their heads off.

Essentially, a continuation is the abstract notion of "what do do next".

Languages like Scheme carry one of these around on each of their stack frames as something accessible from the language itself. call-with-current-continuation is simply a Higher Order Functionish way of doing this.

  ;; Guile scheme
  (call-with-current-continuation
     ;; Often abbr'd to call-cc. Takes a function as arg.
     (lambda (fred)
        (+ 3 4)
        (do-something 42 fred)
        (fred 42) ))
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What's happening here is that when the outer call-cc is run, it takes the continuation of its current context -- where the call-cc would go next if it exited normally -- and calls (lambda (fred) ...) with the continuation in the formal parameter 'fred'.

When the function object in fred is called, the call-cc exits, and the interpreter goes merrily on its way. The call-cc call itself evaluates to whatever is passed to 'fred' when (or if) that gets called.

Note that the function in 'fred' can be passed around too, and called by other functions. The do-something function used above might decide to call its 'fred' argument with some answer other than 42.

You could fake Scheme's call-cc behaviour in Perl using an eval-BLOCK/if($@){} combo. The trick is distinguishing our fake death from a real thrown error:

sub call_cc (&)
{
        my $clause = shift;
        my @fake_returns;
        my $fake_continuation;
        $fake_continuation = sub {
                @fake_returns = @_;
                die $fake_continuation;
        };
        eval {
                return $clause->($fake_continuation);
        };
        if ($@)
        {
                die $@  unless $@ == $fake_continuation;
                return @fake_returns;
        }
        #NOTREACHED
}

print STDERR call_cc {
        my $fred = shift;
        $fred->(42);
        return 64;  # NOTREACHED
};
print STDERR "\n";
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But no, it's not a real continuation.

In Section Seekers of Perl Wisdom