###
### lookfor-continuation
###

## Chapter 8 section 8.1

sub lookfor {
  my $wanted = shift;
  my $value = shift || sub { $_[0] };
  my $u = shift;
  $wanted = [$wanted] unless ref $wanted;

  my $parser = parser {
    my ($input, $continuation) = @_;
    return unless defined $input;

    my $next = head($input);
    for my $i (0 .. $#$wanted) {
      next unless defined $wanted->[$i];
      return unless $wanted->[$i] eq $next->[$i];
    }
    my $wanted_value = $value->($next, $u);

    # Try continuation
    if (my ($v) = $continuation->(tail($input))) {
      return $wanted_value;
    } else {
      return;
    }
  };

  $N{$parser} = "[@$wanted]";
  return $parser;
}