sub expand {
    my $macros = shift;
    my $result;
    my @errors;
    for ( keys %$macros ) {
        $result->{$_} = eval $macros->{$_};
        push @errors, $@ if $@;
    }
    if ( @errors ) {
        $@ = join "\n", @errors;
        return;
    }
    $result;
}