Re^2: "Safe" ways for Carp to report errors to browser?

Of course! "Doh!" on me...

FWIW, the reason I overlooked this might be a common one that others (doing a search) might also overlook, so I think it deserves a tad clarification --please correct me if I'm wrong on this:

"require" waits until runtime to bring the code in, unlike "use", which compiles the function into place before running. Hence, using "use" as I did causes the fatalsToBrowser() function to be called irrespective of the conditional in the if statement. The solution you showed, using the "require"/"import" combination, assures that the code doesn't get imported and used until runtime, allowing the conditional will be tested beforehand.

The reason this eluded me in the first place is because this is all taking place within a BEGIN block, which is executed at compile-time, not runtime. Here, it would have seemed irrelevant whether to use use or require, till I realized that all I'm doing is defining the function, set_message(). This function isn't getting called yet--it's just being defined. Later, if there are syntax errors that perl's compile phase will catch, then set_message() is called, which will then do the test, which then determines whether to imports fatalsToBrowser().

If my explanation is accurate, it could be useful for people who are looking to learn the subtle nuances between use and require.

Dan Heller
http://www.danheller.com

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Re^3: "Safe" ways for Carp to report errors to browser? by dave_the_m (Monsignor) on Jan 10, 2005 at 19:46 UTC
Well, I don't quite follow your explanation (I'm not sure where set_message() appears - it's not in your sample code), so instead I'll just describe what happens. When perl gets to the end of compiling a BEGIN block, it temporarily stops compiling, executes the code in the block, then continues compiling. Similarly, when Perl sees a 'use' while compiling, it temporarily stops the current compile, reads in the file referenced by the 'use', compiles it, executes it, calls the import function(), then continues off compiling at the statement following the 'use'. In your original code, the Perl interpreter starts compiling the code in your BEGIN block, sees the 'use', reads in and compiles the file, calls its import method, then continues compiling the rest of the code in the BEGIN block. (At this point, the import has already affected the current package's symbol table for any subsequent code). Finally the end of the BEGIN block is reached, compilation pauses and the BEGIN block is executed. At this point your if statement is executed, which by now has been optimised away to a noop, since it has an empty body (since 'use' is invisible at runtime). Dave.	[reply] [d/l]
Re^4: "Safe" ways for Carp to report errors to browser? by argv (Pilgrim) on Jan 10, 2005 at 22:41 UTC
You said... Well, I don't quite follow your explanation (I'm not sure where set_message() appears - it's not in your sample code), so instead I'll just describe what happens. Ooops... I forgot that I provided a code snippet rather than my "real" code... I started with the template that's in the CGI::Carp doc, which illustrates how to use `set_message()` by setting a function. In my live code, I use an if statement to test if the browser has a cookie that says whether to output error text, but forgot to include it in my code segment (to keep it brief). That's what I was referring to in my reply that you couldn't reconcile. Anyway, your explanation was perfectly clear, and confirms my explanation as well. I will include my entire `BEGIN` block in a separate posting (which you can find in posting #421127), which takes a slightly different tack than my question here. The reason is that I have a separate, unrelated question. Dan Heller http://www.danheller.com	[reply] [d/l] [select]