Perl Best Practices

13.10. Volatile Error Messages

Use exception objects when error messages may change.

In a string-based exception, the error message is the exception. That can lead to problems during development or maintenance, because it means that any exception handler's ability to recognize a string-based exception is inextricably tied to the structure of the error message itself.

If you ever need to change an exception message in any way, you're going to have to check and update every place that error might ever be caught. In practice, that means that you can never change the text of any exception once the code that throws it is in production^[*].

^[*] Which is why Perl's own built-in exceptions nowadays find themselves in a maze of twisty grammatical forms, variant spellings, and inconsistent capitalizations...none alike.

In contrast, the error message of an object-oriented exception is merely one attribute of that object. More importantly, that message no longer defines the identity and type of the exception. That defining role is now played by the class into which the exception is blessed, or more particularly, by the caught( ) method that the class provides.

So the error message of an exception object can be rewritten whenever necessary. Provided the class of the exception remains the same, any exception handlers that catch it will be unaffected by the change of message.