Advanced Oracle PL/SQL Programming with Packages

3.7. Handling Program Assumptions

Now let's address the problem of invalid action codes. You've already seen the downside: the user is not notified of an invalid entry; the program simply failed with -6503. With my latest version of twice, you no longer get the error. On the other hand, the function now returns the same value if you pass in a NULL string or if you pass in a bad action code. This is not a good way for a function to notify the user of errors. And that is because I do not explicitly handle an underlying assumption of my program.

Just about every piece of software you write makes assumptions about the data it manipulates. For example, parameters may have only certain values or must be within a certain range; a string value should have a certain format, or perhaps an underlying data structure is assumed to have been created. It's fine to have such rules and assumptions, but it is also very important to verify, or assert, that none of the rules are being violated. Because if you assume it and you don't check, your program can end up acting very strangely.

In the twice function, I assume that you, the user, know that you use UL for UPPER-lower, LU for lower-UPPER, and N for no case conversion. But how are you supposed to know this? You either have to see the source code, which is not always going to be possible or desirable, or you have to be given external documentation about the function. And even if you read the documentation on Monday, who says you are going ...