DEFINITION MODULE Stack;



  CLASS Stack(Element);

    REDEFINE METHOD CREATE;

    METHOD Push(El: Element);

    METHOD Pop: Element;

    METHOD Top: Element;

    METHOD Drop;

    METHOD Dup;

    METHOD Swap;

    METHOD Depth: INTEGER;

  END Stack;



END Stack;

When creating a new instance of such a class, an actual class must be provided for each formal class. For instance, given the Stack declaration above, one must indicate the type of the elements which are contained by a Stack instance:

VAR

  RectStack: Stack(Rectangle);

YAFL limits the usage of parameterized classes by requiring the actual parameters to be classes, as opposed to a predefined type such as integer, boolean or real. Such a design decision simplifies implementation significantly and guarantees excellent performance.

If one wishes to use predefined types as actual parameter to a parameterized class, YAFL's standard library includes an AbstractClasses module which declares classes which behave as syntactic sugar around predefined types.

ONCE classes

A class marked as ONCE denotes a single instance, created automatically at initialization time. ONCE classes can be used to describe unique resources such as the standard input and output streams, a semaphore, or as recipiendary for conventional function libraries:

DEFINITION MODULE Trig;

  ONCE CLASS Trig;

    METHOD Sin(r: REAL): REAL;

    METHOD Cos(r: REAL): REAL;

  END Trig;       -- End of class

END Trig;         -- End of module

ONCE class must fulfill three constraints:

• It cannot be inherited
• It cannot be parameterized
• Its CREATE method must be parameterless