1. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 1 of 14Exception Handling in PerlPresented by Ian KluftSilicon Valley PerlJuly 7, 2011Santa Clara, California

2. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 2 of 14Exception Handling in PerlThree parts to this presentation What is exception handling? Exception handling modules for Perl Using exception handling in your Perl code 3. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 3 of 14What is Exception Handling? Method of error handling Exception is an error which can becaught by a calling function Exception unrolls function callsuntil one catches it Contains structured data abouterror, not just numeric code 4. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 4 of 14Exception Handling in the News Dont forget to catch exceptions in your code Highest profile example: Ariane 501 accident June 4, 1996 in Kourou, French Guiana ESA space rocket exploded 36 seconds into flight Accident investigators found software crashed ontest bench 36 seconds into simulated flight (!!!) Integer counter overflow exception was not caught Software was only tested with Ariane 4 flight profile Ariane 5 overflowed a horizontal motion counter 5. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 5 of 14Causes of Exceptions Exceptions may be caused by fatal error incode or system call If the program signals an exception itself, it iscalled throwing an exception Most Perl code uses simple form of throwingexceptionsopen FILE, foo or die open failed: $! 6. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 6 of 14Mechanism of Catching Exceptions Exceptions can be caught in Perl with eval() This is just showing you how it works Dont re-invent the wheel There are modules for thiseval {... code to do something ...};if ($@) {handle_error($@);} 7. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 7 of 14Exception Handling ModulesCPAN has many exception handling modules Exception::Class is recommended, shown here Exception::Lite also currently maintained Error adds try/catch style to Perl syntax Exception::System catch system call errors Exception (not currently maintained) Class::Throwable (not currently maintained) Some module sets use own exception handling 8. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 8 of 14Exception::Class Exceptions are classes inheriting fromException::Class or your subclasses of it You can organize exceptions hierarchically Exception::Class::Base offers handler code Using classes sets exceptions at compile time 9. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 9 of 14Exception::Class declarationFrom manual page:use Exception::Class( MyException,AnotherException =>{ isa => MyException },YetAnotherException =>{ isa => AnotherException,description => These exceptions are related to IPC },ExceptionWithFields =>{ isa => YetAnotherException,fields => [ grandiosity, quixotic ],alias => throw_fields,},); 10. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 10 of 14Exception::Class usageFrom manual page:# tryeval { MyException->throw( error => I feel funny. ) };my $e;# catchif ( $e = Exception::Class->caught(MyException) ) {warn $e->error, "n", $e->trace->as_string, "n";warn join, $e->euid, $e->egid, $e->uid, $e->gid,$e->pid, $e->time;exit;} elsif ( $e = Exception::Class->caught(ExceptionWithFields) ) {$e->quixotic ? do_something_wacky() : do_something_sane();} else {$e = Exception::Class->caught();ref $e ? $e->rethrow : die $e;} 11. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 11 of 14Wrapping main in exception handler Good practice... no! This is a best practice. To catch all exceptions, make main() anexception handler wrapper Adapt manual page code from previous slide Catch Exception::Class exceptions Then report unknown errors, like die() More graceful exit for program 12. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 12 of 14Building depth into error reporting Exception classes can be configured to collectstack traces Data structures can be passed from errorrecognition point to error reporting code Exception::Class::DBI integrates w/ DBI code Catch unexpected database errors Make a sane error report to user Mail notification about database errors to DBAs 13. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 13 of 14Exceptions with text descriptions All errors can and should have text descriptions Error reporting and documentation can bemuch more organized Extract error documentation from Perl code Document for engineers what causes that error Document for user/operator what to do whenthey see that error Document for support dept what it means whencustomer calls with that error 14. Presented at SVPerlJuly 7, 2011Exception Handling in PerlBy Ian KluftSlide 14 of 14Perl Exception Handling: go do it! Everything is easier when you design it in But this can be added to existing code First, add exception-handler wrapper(s) Then, define exception classes as needed Replace error codes with throwing exceptions Both can exist during transition Goal: get rid of numeric error codes!