sub is_string
{
    return sub {
        return $_[0] ne "";
    };
}

sub is_number
{
    return sub {
        return $_[0] =~ m{^\d+$};
    };
}

sub is_range($$)
{
    (my $l, my $u) = @_;
    return sub {
        return $_[0] =~ m{^\d+$} && $_[0] >= $l && $_[0] <= $u;
    };
}

sub is_in_list($)
{
    my $l = shift;
    return sub {
        return (grep { $_[0] eq $_ } @{$l}) != 0;
    };
}

sub is_email
{
    return sub {
        return $_[0] =~ m{^[-.+a-z0-9]+\@[-.+a-z0-9]+$}i;
    };
}

sub is_required
{
    my $t = shift;
    return sub {
        defined $_[0] and &{$t}($_[0]);
    };
}

sub is_optional
{
    my $t = shift;
    return sub {
        return defined $_[0] ? &{$t}($_[0]) : 1;
    };
}

sub is_equal
{
    my $v = shift;
    return sub {
        $_[0] eq $v ? 1 : 0;
    }
}