Revealing Methods in Interfaces

INTERFACE Directory;

TYPE
  (* The implementation of Directory.T supports the
     following methods: init, m
     all other details are hidden from clients *)
  PublicT = BRANDED OBJECT METHODS
              init() : PublicT := Init;
	      add();
            END;
  T <: PublicT;

PROCEDURE Init (t: PublicT) : PublicT;

END Directory.

(**************************)

INTERFACE FileSystemDirectory;
IMPORT Directory;

TYPE
  (* brand PublicT so that a client cannot fake
     a revelation.
     if this type were not BRANDED, someone
     without the right to see this interface
     could state their own partial revelation
     for Directory.T *)
  PublicT = Directory.T BRANDED OBJECT METHODS
              (* do not call this init
                 if you do, then you cannot override
                 the Directory.T.init method *)
	      initFSD() : T;
            END;
  T <: PublicT;

END FileSystemDirectory.

(**************************)

MODULE FileSystemDirectory;

IMPORT Directory;

REVEAL
 (* T is a subtype of Directory.T *)
 T = PublicT BRANDED OBJECT OVERRIDES
               (* override any superclass methods
                  that could be dangerous *)
               init := Init;
               initFSD := InitFSD;
               add := Add;
             END;

(* the initialization method for T *)
PROCEDURE InitFSD (t: T) : T =
  BEGIN
    (* call initialization on the superclass *)
    RETURN Directory.T.init(t);
  END InitFSD;

(* cancel the inherited Init method *)
PROCEDURE Init (t: T) : Directory;
  BEGIN
    (* do nothing *)
    RETURN t;
  END Init.

BEGIN
END FileSystemDirectory.