User-defined Types

var YType := class Value { var dynamic type;

var init := method [ t ] { type:= t ; return this};

var check := method t {return type <= t.getType[]}; };

var Persons := class Value {var msg := "I am a person.\n"};

var Staff := class Persons {var msg := "I am a staff.\n"};

var YStaff := new YType[Staff];

var s:YStaff; // Upwards-closure type

s := new Persons; print s.msg;

var p:Staff; // Normal type requires exact match.

p := new Persons; print s.msg;