sub alternate {
	my $iter = shift;
	my $loop = shift; # main loop callback
	my @conds;
	while (@_ > 0) {
		# sub to get value, sub to call if value changed
		push @conds, [ shift, shift ];
	}

	my @memory;
	return unless $iter->();
	# starting values
	@memory = map { $_->[0]->() } @conds;

	do {
		$loop->();

		while (my ($idx, $cond) = each @conds) {
			my $new = $cond->[0]->();

			next if $memory[$idx] eq $new;

			$cond->[1]->(); # callback

			$memory[$idx] = $new;
			# reset subsequent values
			$memory[$_] = $conds[$_][0]->() for ($idx + 1 .. $#conds);

			last;
		}
	} while ($iter->());
}

my $gen = make_generator(\my ($day, $type, $word));

alternate(
	$gen,
	sub { print "$word, " },
	sub { $day },
	sub { print "</td></tr>\n<tr><td>$day</td><td>" },
	sub { $type },
	sub { print "</td><td>" }
);