Optional procedure parameter

GNU Modula-2 allows the last parameter to a procedure or function parameter to be optional. For example in the ISO library COROUTINES.def the procedure NEWCOROUTINE is defined as having an optional fifth argument (initProtection) which, if absent, is automatically replaced by NIL.

NEWCOROUTINE
PROCEDURE NEWCOROUTINE (procBody: PROC; workspace: SYSTEM.ADDRESS;
                        size: CARDINAL; VAR cr: COROUTINE;
                        [initProtection: PROTECTION = NIL]);

  (* Creates a new coroutine whose body is given by procBody,
     and returns the identity of the coroutine in cr.
     workspace is a pointer to the work space allocated to
     the coroutine; size specifies the size of this workspace
     in terms of SYSTEM.LOC.

     The optional fifth argument may contain a single parameter
     which specifies the initial protection level of the coroutine.
  *)

The implementation module COROUTINES.mod implements this procedure using the following syntax:

PROCEDURE NEWCOROUTINE (procBody: PROC; workspace: SYSTEM.ADDRESS;
                        size: CARDINAL; VAR cr: COROUTINE;
                        [initProtection: PROTECTION]);
BEGIN

END NEWCOROUTINE ;

Note that it is illegal for this declaration to contain an initialiser value for initProtection. However it is necessary to surround this parameter with the brackets [ and ]. This serves to remind the programmer that the last parameter was declared as optional in the definition module.

Local procedures can be declared to have an optional final parameter in which case the initializer is mandatory in the implementation or program module.

GNU Modula-2 also provides additional fixed sized data types which are all exported from the SYSTEM module. See The PIM system module. See The ISO system module.