sub enum {
    my ($attr, $value, $constraints) = @_;

    my $enum_values = $constraints->{enum};

    if (scalar @$enum_values) {
        if (! (grep /^$value$/, @$enum_values)) {
            my $enum_values_string = join ', ', @$enum_values;
            return "Attribute $attr is enum type but value '$value' isn't one of '$enum_values_string'";
        }
    }

    return 0;
}