Forward Procedures

[Helpdesk]

How do I declare a forward Procedure? The declaration:

Code: Select all

PROCEDURE ^P(i: INTEGER);

Results in the error message "no ;" at the ^.

[cfbsoftware]

There is no special mechanism used for forward procedures in Oberon-07. Normally procedures should be declared before they are referenced. The only time where it is necessary to use a forward procedure is if two procedures mutually call each other e.g. if you want to do something like:

Code: Select all

PROCEDURE ProcA(a: INTEGER);
BEGIN
  ...
  ProcB(a-1);
  ...
END ProcA;

PROCEDURE ProcB(a: INTEGER);
BEGIN
  ...
  ProcA(a-1);
  ...
END ProcB;

There are two ways you can achieve this:

a) Nest one procedure inside the other:

Code: Select all

PROCEDURE ProcA(a: INTEGER);

  PROCEDURE ProcB(a: INTEGER);
  BEGIN
    ...
    ProcA(a-1);
    ...
  END ProcB;

BEGIN
  ...
  ProcB(a-1);
  ...
END ProcA;

b) Declare one of the procedures as procedure variable:

Code: Select all

VAR
  procVarB: PROCEDURE(a: INTEGER);

PROCEDURE ProcA(a: INTEGER);
BEGIN
  ...
  procVarB(a-1);
  ...
END ProcA;

PROCEDURE ProcB(a: INTEGER);
BEGIN
  ...
  ProcA(a-1);
  ...
END ProcB;

BEGIN
  procVarB := ProcB
END.