I don't know why the “parametrizable” never registered regardless of my repeated mention.

1. How are you going to define a type for reals in the range 0.0 to 1.0?
2. And another for those in the range -1.0 to +1.0?
3. And another for integers that can be (-1|0|+1)?
4. And another for dates this year? This month? This century?
5. And another for daytimes? Nighttimes? Weekends? First days of the month?
6. And one for 3-char strings? and 4-chars strings? and 5-char strings? And lowercase 3-char strings? and uppercase 3-char strings? And lowercase, 3-char strings that begin with 'a'? And lowercase 3-char strings that begin with 'b'? …or 'c'? …or 'd'?

1. my $foo is Real( 0.0, 1.0 );
2. my $foo is Real( -1.0, 1.0 );
3. my $foo is InSet( -1, 0, -1 );
4. my $foo is DateTime( year => DateTime.now.truncate( to => 'year' ) ); # ...
5. my $foo is DateTime( hour => [ 8 .. 22 ] ); # ...
6. my $foo is String( len => 3 ); # ...
   my $foo is String( match => /\A a [[:lower:]]{2} \z/ ); # ...

For the Real constructor I could even imagine some syntax like qw( ] 0.0 - 1.0 ] ) (I don't remember which dingbats we'll use in Perl6 for qw//) which would define a range from 0.0 exclusive to 1.0 inclusive — just like the mathematical notation.

Use your imagination. Did I mention declarative programming is bliss for maintainers?

Makeshifts last the longest.