Sub-typing (Subsumption)

• Syntax:

  TYPE type_name SUBSUMES type = [type_declaration_signature];
  

• Description:

  A sub-type is obtained by restricting the set of possible values an object of that type can take.

  Examples:

  TYPE GenericIntervalType =
  RECORD
  {
    LowerBound: Generic;
    UpperBound: Generic;
  };
  

  The "Generic" type is a built-in type that unifies with any other type; any type is a sub-type of "Generic".

  TYPE RealIntervalType "Interval of real numbers"
  SUBSUMES GenericIntervalType =
  RECORD
  {
    LowerBound: Real; // Real is a sub-type of Generic
    UpperBound: Real;
  };
  

  "RealIntervalType" is a sub-type of "GenericIntervalType".