Or unless you have a type "Non-negative number".

Which you can define easily in Perl 6 :):

  my subtype NonNegativeNumber of Num where { not $^a < 0 };

  my NonNegativeNumber $x =  3; # works
  my NonNegativeNumber $y = -3; # dies
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Another example of using subtypes:

  my subtype HTTP::Request::Method of Str where {
    $^method eq any <GET POST HEAD ...>;
  };

  method send_using(HTTP::Request::Method $method) {
    ...;
    # Note: You don't have to manually check for $method
    # not being a valid HTTP method :).
  }
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--Ingo