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NAME ^

App::exceptions - Programming with Exceptions

INTRODUCTION ^

Programming with Exceptions is a much more reliable way to prepare for unusual conditions than trying to handle error codes returned from functions and methods That is why programming with exceptions is an Enterprise Programming topic. However, Perl has not always supported programming with exceptions, and many Perl programmers are not familiar with this style of programming using Perl.

This document was written to explain the support within the Perl language for programming using exceptions in general. It also explains the standards for exception programming in the App-Context framework.

The short answer is that we recommend the following modules to assist in exception programming.

  Carp
  CGI::Carp
  Fatal
  Exception::Class
  Devel::Stacktrace

However, it is important to understand what each does for you, why, and how they all fit together. So for the longer answer, read on.

IMPORTANCE OF EXCEPTION PROGRAMMING ^

Exception Programming is a technique to deal with exceptional conditions (i.e. errors) encountered during runtime. It is contrasted with the more common technique of Error Checking.

Error Checking is a technique where an operation is performed. Then all possible error conditions are checked and handled. This requires the developer to address the following challenges.

1. Error Completeness

The developer must consider all possible error conditions (difficult to foresee and consider at every level).

2. Error Propagation

The developer must add quite a bit of additional logic to allow subsequent statements after the error condition to be skipped and the error to be propagated upward in the call stack to the place where it can be acted on appropriately.

The problems with Error Checking is that it is so difficult to do and so rarely done completely. Since the program runs fine when no errors are encountered, the developer invariably fails to put in all of the error checks and the accompanying error propagation logic.

Exception Programming addresses these problems using the following methods.

1. Error Completeness

Exceptions are grouped into an exception hierarchy. Therefore, high level code needs only make the distinctions between exceptions that it thinks are relevant. If additional exceptions are added later by low level code, they will be handled in accordance with the upper levels of the exception hierarchy which are already known.

2. Error Propagation

Exceptions take advantage of built-in language support (longjmp() in C) to unravel the call stack. Additional logic is not required by the developer to propagate the error to a place that is prepared to handle it.

Because Exception Programming addresses the issue of software reliability (one of the Attributes of Enterprise Systems), it is necessarily an Enterprise Programming issue.

PERL 5 LANGUAGE SUPPORT ^

Perl did not always support exception programming. Even in Perl 5.6.1, the support for exception programming is somewhat scattered. The following are features of Perl 5 which are relevant to exception programming.

die

  Camel Book: Chapter 3: Functions, "die"
  Online Doc: man perlfunc (or "perldoc perlfunc")

The "die" function prints its arguments to STDERR and causes the program to exit. It has some extra features, like appending __FILE__ and __LINE__ strings if the args don't end with a newline. The biggest feature is that it may be "caught" if it is executed within an eval statement.

Simply put, "die" is the native Perl way of "throwing" an exception. The only "attributes" of the "exception" is the message (concatenated args) of the "die" itself.

  if ($error_condition_xyz_exists) {
      die "Error XYZ occurred";
  }

The "warn" function is similar to "die", in that it prints out its arguments to STDERR in the same way, but it does not try to exit or throw an exception in any way. Therefore, it's not relevant for this discussion.

  die  "I'm dying";    # print msg to STDERR and exit

croak and confess

  Camel Book: Chapter 7: The Standard Perl Library, "Carp"
  Online Doc: man Carp (or "perldoc Carp")

The Carp.pm module is part of the Standard Perl Library It allows you to throw exceptions (like "die"), but it reports the line number and file name from the perspective of the caller of the function/method that died.

This is useful for library code, so that the programmer sees where he invoked the function that failed rather than seeing where within that function the failure occurred.

  use Carp;

  croak "We're outta here!";     # like "die", but line # from caller
  confess "It was my fault: $!"; # like "croak" but with stack trace

Using "croak" to throw an exception references the line # and file name of the caller.

Using "confess" to throw an exception causes a stack trace to be printed also.

CGI::Carp

  Online Doc: man CGI::Carp (or "perldoc CGI::Carp")

If you are developing perl scripts which will be run by a web server, the STDERR stream is usually redirected the web server error log. However, the output generated by "die" is not formatted nicely, with a datetime stamp and filename, as is customary with error log entries.

The CGI::Carp package replaces die, croak, and confess with versions which work the same but format the output a little more nicely for the web server error log. You can even direct fatal errors to the browser.

  use CGI::Carp;

  die  "I'm dying";              # print msg to log and exit
  warn "I'm confused";           # print msg to log
  croak "We're outta here!";     # like "die", but line # from caller
  confess "It was my fault: $!"; # like "croak" but with stack trace
  carp "It was your fault!";     # like "warn", but line # from caller

  use CGI::Carp qw(fatalsToBrowser);
  die "Fatal error messages are now sent to browser";

$!

  Camel Book: Chapter 2: The Gory Details/Special Variables, $!
  Online Doc: man perlvar (or "perldoc perlvar")

If used in a string context, yields the error string from the last system call error in the currently executing perl interpreter.

You shouldn't depend on $! being anything in particular unless you've gotten a specific error indicating a system error.

So when you get a system call error returned from a perl function (system calls usually relate to files, networking, processes, or interprocess communication), you can check $!.

If you want to use good exception programming techniques, every perl function which can fail from a system call error should be checked for its return value so that an exception may be thrown.

  $file = "foo.txt";
  open(FILE, "< $file") || die "Error opening [$file]: $!";

Of course, if your program's requirements allow you to handle the error locally without throwing an exception, you are welcome to. However, every possible error needs to be thought about and accounted for.

  $file = "foo.txt";
  if (open(FILE, "< $file")) {  # if file doesn't exist, that's ok
      @data = <FILE>;
      close(FILE);
  }

eval

  Camel Book: Chapter 3: Functions, "eval"
  Online Doc: man perlfunc (or "perldoc perlfunc")

Eval serves as Perl's version of a "try {}" block.

If a "die" (an exception) occurs within an "eval", the program is not terminated, only the "eval". All of the code that was eval'ed, after the exception, is skipped.

Eval has two syntaxes, "eval EXPR" and "eval BLOCK".

The "eval EXPR" syntax causes the code contained in the EXPR expression to be compiled every time the statement is executed during runtime.

  eval "&do_big_function();";
  $code = "&do_another_big_operation();";
  eval $code;

The "eval BLOCK" syntax causes the code contained in the BLOCK to be compiled only once (at script compile time), so it is much more efficient and appropriate for exception programming. (Please note the semi-colon that must follow the BLOCK.)

  eval {
      &do_big_function();
  };

If any exception ("die") is thrown within the do_big_function(), the program will not terminate. Control will simply be returned to the end of the eval block.

$@

  Camel Book: Chapter 2: The Gory Details/Special Variables, $@
  Online Doc: man perlvar (or "perldoc perlvar")

The is the Perl syntax error message from the last "eval" command. Alternatively, it is the message of an exception that was thrown (using "die"). If null, the last eval was parsed and executed correctly, with no exceptions.

Thus, perl exception programming can be done with the following.

  eval {
      &do_big_function();
  };
  if ($@) {
      # handle the exception ($@ is the exception message)
  }

  sub do_big_function {
      ...
      if ($error_condition_xyz_exists) {
          die "Error XYZ occurred";
      }
      ...
  }

$SIG{__DIE__}

  Camel Book: Chapter 2: The Gory Details/Special Variables, %SIG
  Online Doc: man perlvar (or "perldoc perlvar")

This handler is not necessary to know about to do exception programming in Perl. However, it may be used by modules that assist in exception handling tasks.

The %SIG hash contains references to subroutines which are called in response to signals received by the process. "__DIE__" is a special internal hook which is not really an external signal.

So if you desired to do some additional processing between the time that the "die" exception was thrown and the time it was handled, you could replace the $SIG{__DIE__} handler.

  $SIG{__DIE__} = sub {
      # do something (@_ are the args of the "die")
  };

caller

  Camel Book: Chapter 3: Functions, "caller"
  Online Doc: man perlfunc (or "perldoc perlfunc")

The "caller" function allows you to examine the call stack in order (for example) to print stack traces.

Weaknesses in the Native Perl Exception Support

The eval/die pair provides the critical language support necessary for Error Propagation.

However, the single attribute of an exception, the message, leaves the developer without an exception hierarchy which would allow him to ensure Error Completeness in his exception handling.

Furthermore, the Perl functions themselves must be checked for errors rather than throwing exceptions.

EXCEPTION PROGRAMMING USING Fatal.pm ^

Causes all of the perl functions to throw exceptions rather than simply return error codes or set error flags.

EXCEPTION PROGRAMMING USING Error.pm ^

Introduces the ability to have an Exception Class (not just a message) and to use "try/catch" syntax.

It does this be defining try() and catch() subroutines with appropriate prototypes, using the "{}" blocks as closures (or anonymous subroutines). See the following references.

  Camel Book: Chapter 2: The Gory Details, Subroutines, Prototypes
  Camel Book: Chapter 4: References and Nested Data Structures, Using Hard References, Closures

However, you must be very careful in the placement of your semi-colons using the try/catch syntax provided by Error.pm or your module will leak memory (very bad in a mod_perl context).

EXCEPTION PROGRAMMING USING Exception::Class ^

Introduces the capability to collect exception classes easily in a class hierarchy. This helps address the issue of Error Completeness.

EXCEPTION PROGRAMMING USING Devel::Stacktrace ^

Adds the capability to print out more detailed stack trace information than was possible with "confess".

FURTHER RESEARCH ^

An excellent section on Exception Handling in Perl exists in the mod_perl Guide.

  http://perl.apache.org/guide/perl.html#Exception_Handling_for_mod_perl

You can read up on future directions for Perl and the evolving exception support here.

  http://www.perl.com/pub/a/2002/01/15/apo4.html?page=3
  http://dev.perl.org/rfc/88.html
  http://nntp.perl.org/group/perl.perl6.language.errors
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