start page | rating of books | rating of authors | reviews | copyrights

Oracle PL/SQL Programming, 2nd Edition

Oracle PL/SQL Programming, 2nd EditionSearch this book
Previous: 15.8 Module Overloading Chapter 15
Procedures and Functions
Next: 15.10 Go Forth and Modularize!
 

15.9 Forward Declarations

PL/SQL is rather fussy about its requirement that you declare a variable, cursor, or module before you use it in your code. Otherwise, how can PL/SQL be sure that the way you are using the object is appropriate? Since modules can call other modules, however, you can encounter situations where it is completely impossible to define all modules before any references to those modules are made. What if program A calls program B and program B calls program A? PL/SQL supports mutual recursion, where two or more programs directly or indirectly call each other.

If you do find yourself committed to mutual recursion, you will be very glad to hear that PL/SQL supports the forward declaration of local modules, which declares a module in advance of the actual definition of that program. This declaration makes that program available to be called by other programs even before the program definition.

Remember that both procedures and functions have a header and a body. A forward declaration consists simply of the program header, followed by a semicolon ( ; ). This construction is called the module header. This header, which must include the parameter list (and RETURN clause if a function) is all the information PL/SQL needs about a module in order to declare it and resolve any references to that module; a module should, after all, be a little black box.

The following example will illustrate the technique. I define two mutually recursive functions within a procedure. Consequently I have to declare just the header of my second function, total_cost, before the full declaration of net_profit:

PROCEDURE perform_calcs (year_in IN INTEGER) IS    /* Header only for total_cost function. */    FUNCTION total_cost (. . .)  RETURN NUMBER;     /* The net_profit function uses total_cost. */    FUNCTION net_profit (. . .) RETURN NUMBER    IS    BEGIN       RETURN tot_sales (. . .) - total_cost (. . .);    END;     /* The total_cost function uses net_profit. */    FUNCTION total_cost (. . .)  RETURN NUMBER    IS    BEGIN       IF net_profit (. . .) < 0       THEN          RETURN 0;       ELSE          RETURN . . .;       END IF;    END; BEGIN    . . . END;

Here are some rules to remember concerning forward declarations:

In some situations, you absolutely require forward declarations; in most situations, they just help make your code more readable and presentable. As with every other advanced or unusual feature of the PL/SQL language, use forward declarations only when you really need the functionality. Otherwise, the declarations simply add to the clutter of your program, which is the last thing you want.


Previous: 15.8 Module Overloading Oracle PL/SQL Programming, 2nd Edition Next: 15.10 Go Forth and Modularize!
15.8 Module Overloading Book Index 15.10 Go Forth and Modularize!

The Oracle Library Navigation

Copyright (c) 2000 O'Reilly & Associates. All rights reserved.

Library Home Oracle PL/SQL Programming, 2nd. Ed. Guide to Oracle 8i Features Oracle Built-in Packages Advanced PL/SQL Programming with Packages Oracle Web Applications Oracle PL/SQL Language Pocket Reference Oracle PL/SQL Built-ins Pocket Reference