Forward Declarations
The PL/SQL needs that you declare an identifier before using it. And hence, you should declare a subprogram before calling it. For illustration, the declaration below of the procedure award_bonus is illegal as the award_bonus calls the procedure calc_ rating that is not yet declared when the call is made:
DECLARE
...
PROCEDURE award_bonus ( ... ) IS
BEGIN
calc_rating( ... ); -- undeclared identifier
...
END;
PROCEDURE calc_rating ( ... ) IS
BEGIN
...
END;
In this situation, you can solve the problem easily by placing the procedure calc_rating before procedure award_bonus. Though, the easy solution does not always work. For illustration, assume that the procedures are mutually recursive or you want to define them in the alphabetical order. The PL/SQL solves the problem by providing a special subprogram declaration known as the forward declaration. You can use the forward declarations to
(i) Define the subprograms in the logical or alphabetical order.
(ii)Define the mutually recursive subprograms.
(iii)Group the subprograms in a package.
The forward declaration consists of a subprogram specification completed by a semicolon. In the illustration shown below, the forward declaration suggested that the PL/SQL body of the procedure calc_rating can be found later in the block:
DECLARE
PROCEDURE calc_rating ( ... ); -- forward declaration
...
/* Define the subprograms in alphabetical order. */
PROCEDURE award_bonus ( ... ) IS
BEGIN
calc_rating( ... );
...
END;
PROCEDURE calc_rating ( ... ) IS
BEGIN
...
END;
Though the formal parameter list appears in the forward declaration, it should also appear in the subprogram body. You can position the subprogram body anywhere after the forward declaration, but they should appear in the same program unit.