test.pl

use Mojo::Base -strict;

use Test::More;
use Carp ();
use Mojo::Promise;

sub await;

sub async {
  my $body = shift;
  my $wrapper = sub {
    local *await = sub {
      my $p = shift;
      my (@retvals, $err);
      $p->then(sub{
          @retvals = @_;
          goto DONE;
        }, 
        sub {
          $err = shift;
          goto DONE;
        }
      );
      goto START;
      DONE: {
        Carp::croak($err) if $err;
        return wantarray ? @retvals : $retvals[0];
      };
    };
    my $p = Mojo::Promise->new;
    eval { $p->resolve($body->(@_)); 1 } or $p->reject($@);
    START: return $p;
  };
  return $wrapper;
}

sub double {
  my $in = shift;
  my $p  = Mojo::Promise->new;
  Mojo::IOLoop->timer(0.5 => sub { $p->resolve(2 * $in) });
  return $p;
}

my $tick = 0;
Mojo::IOLoop->recurring(0.1 => sub { $tick++ });

my $answer;
async(sub { $answer = await double(21) })->()->wait;

Mojo::IOLoop->timer(
  5 => sub {
    fail 'timeout';
    Mojo::IOLoop->stop;
  }
);

is $answer, 42, 'got expected answer';
ok $tick > 2, 'got multiple ticks';

done_testing;