yea I hear you. The problem I have will passing the http parameters is that it only goes one level deep. If you want to nest it arbitrarily you would have to build up a comma separated list of values to refer to all the places you need to return to. At that point, if you think of this list of return places as a set of pointers it is as if you are implementing a stack. Having this kind of thing as a first class construct could maybe allow this to be checked by the compiler.

To be honest the details of implementing 'scalable multi-threaded continuations' is beyond my skill level.

Yes scaling continuations based web apps can be a big headache. I think this is why the Seaside developers have been scaling back continuations and making them optional in the framework: http://lists.squeakfoundation.org/pipermail/seaside/2007-Dec...

Coroutines are an interesting lighter alternative (though will still suffer from threading & session affinity issues).

See coro based libraries/frameworks influenced by Seaside like Continuity (http://continuity.tlt42.org/) & Nagare (http://www.nagare.org/) (which I think uses coros?).

Here is a fulling working Continuity example:

    use strict;
    use warnings;
    use Continuity;
    use HTML::AsSubs;

    Continuity->new->loop;

    sub main {
        my $r = shift;

        ###########################################    
        # helpers

        my $update_billing_info = sub {
            return 1 if eval { $r->param('updated') };
            return;
        };
    
        my $display_update_billing_info = sub {
            $r->print(
                form(
                    input( {type => 'checkbox', name => 'updated'}, 'updated?' ),
                    br,
                    input( {type => 'submit'} ),
                    p( "No. of tries thus far - $_[0]" ),
                )->as_HTML
            );
        };
    
        my $render_checkout_screen = sub {$r->print("Updated after $_[0] submits!")};
    
        ###########################################    
        # business logic

        my $count_tries;
        while (not $update_billing_info->()) {
            $display_update_billing_info->( $count_tries++ );
            $r->next;
        }
    
        $render_checkout_screen->( $count_tries );
    }