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Exception::Class(3)	       User Contributed Perl Documentation	      Exception::Class(3)

NAME
       Exception::Class - A module that allows you to declare real exception classes in Perl

VERSION
       version 1.37

SYNOPSIS
	 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',
	     },
	 );
	 use Scalar::Util qw( blessed );
	 use Try::Tiny;

	 try {
	     MyException->throw( error => 'I feel funny.' );
	 }
	 catch {
	     die $_ unless blessed $_ && $_->can('rethrow');

	     if ( $_->isa('Exception::Class') ) {
		 warn $_->error, "\n", $_->trace->as_string, "\n";
		 warn join ' ', $_->euid, $_->egid, $_->uid, $_->gid, $_->pid, $_->time;

		 exit;
	     }
	     elsif ( $_->isa('ExceptionWithFields') ) {
		 if ( $_->quixotic ) {
		     handle_quixotic_exception();
		 }
		 else {
		     handle_non_quixotic_exception();
		 }
	     }
	     else {
		 $_->rethrow;
	     }
	 };

	 # without Try::Tiny

	 eval { ... };
	 if ( my $e = Exception::Class->caught() ) { ... }

	 # use an alias - without parens subroutine name is checked at
	 # compile time
	 throw_fields error => "No strawberry", grandiosity => "quite a bit";

DESCRIPTION
       RECOMMENDATION 1: If you are writing modern Perl code with Moose or Moo I highly recommend
       using Throwable instead of this module.

       RECOMMENDATION 2: Whether or not you use Throwable, you should use Try::Tiny.

       Exception::Class allows you to declare exception hierarchies in your modules in a "Java-
       esque" manner.

       It features a simple interface allowing programmers to 'declare' exception classes at
       compile time.  It also has a base exception class, Exception::Class::Base, that can be
       easily extended.

       It is designed to make structured exception handling simpler and better by encouraging
       people to use hierarchies of exceptions in their applications, as opposed to a single
       catch-all exception class.

       This module does not implement any try/catch syntax.  Please see the "OTHER EXCEPTION
       MODULES (try/catch syntax)" section for more information on how to get this syntax.

       You will also want to look at the documentation for Exception::Class::Base, which is the
       default base class for all exception objects created by this module.

DECLARING EXCEPTION CLASSES
       Importing "Exception::Class" allows you to automagically create Exception::Class::Base
       subclasses.  You can also create subclasses via the traditional means of defining your own
       subclass with @ISA.  These two methods may be easily combined, so that you could subclass
       an exception class defined via the automagic import, if you desired this.

       The syntax for the magic declarations is as follows:

       'MANDATORY CLASS NAME' => \%optional_hashref

       The hashref may contain the following options:

       o   isa

	   This is the class's parent class.  If this isn't provided then the class name in
	   $Exception::Class::BASE_EXC_CLASS is assumed to be the parent (see below).

	   This parameter lets you create arbitrarily deep class hierarchies.  This can be any
	   other Exception::Class::Base subclass in your declaration or a subclass loaded from a
	   module.

	   To change the default exception class you will need to change the value of
	   $Exception::Class::BASE_EXC_CLASS before calling "import()".  To do this simply do
	   something like this:

	     BEGIN { $Exception::Class::BASE_EXC_CLASS = 'SomeExceptionClass'; }

	   If anyone can come up with a more elegant way to do this please let me know.

	   CAVEAT: If you want to automagically subclass an Exception::Class::Base subclass
	   loaded from a file, then you must compile the class (via use or require or some other
	   magic) before you import "Exception::Class" or you'll get a compile time error.

       o   fields

	   This allows you to define additional attributes for your exception class.  Any field
	   you define can be passed to the "throw()" or "new()" methods as additional parameters
	   for the constructor.  In addition, your exception object will have an accessor method
	   for the fields you define.

	   This parameter can be either a scalar (for a single field) or an array reference if
	   you need to define multiple fields.

	   Fields will be inherited by subclasses.

       o   alias

	   Specifying an alias causes this class to create a subroutine of the specified name in
	   the caller's namespace.  Calling this subroutine is equivalent to calling
	   "<class>->throw(@_)" for the given exception class.

	   Besides convenience, using aliases also allows for additional compile time checking.
	   If the alias is called without parentheses, as in "throw_fields "an error occurred"",
	   then Perl checks for the existence of the "throw_fields()" subroutine at compile time.
	   If instead you do "ExceptionWithFields->throw(...)", then Perl checks the class name
	   at runtime, meaning that typos may sneak through.

       o   description

	   Each exception class has a description method that returns a fixed string.  This
	   should describe the exception class (as opposed to any particular exception object).
	   This may be useful for debugging if you start catching exceptions you weren't
	   expecting (particularly if someone forgot to document them) and you don't understand
	   the error messages.

       The "Exception::Class" magic attempts to detect circular class hierarchies and will die if
       it finds one.  It also detects missing links in a chain, for example if you declare Bar to
       be a subclass of Foo and never declare Foo.

Try::Tiny
       If you are interested in adding try/catch/finally syntactic sugar to your code then I
       recommend you check out Try::Tiny. This is a great module that helps you ignore some of
       the weirdness with "eval" and $@. Here's an example of how the two modules work together:

	 use Exception::Class ( 'My::Exception' );
	 use Scalar::Util qw( blessed );
	 use Try::Tiny;

	 try {
	     might_throw();
	 }
	 catch {
	     if ( blessed $_ && $_->isa('My::Exception') ) {
		 handle_it();
	     }
	     else {
		 die $_;
	     }
	 };

       Note that you cannot use "Exception::Class->caught()" with Try::Tiny.

Catching Exceptions Without Try::Tiny
       "Exception::Class" provides some syntactic sugar for catching exceptions in a safe manner:

	 eval {...};

	 if ( my $e = Exception::Class->caught('My::Error') ) {
	     cleanup();
	     do_something_with_exception($e);
	 }

       The "caught()" method takes a class name and returns an exception object if the last
       thrown exception is of the given class, or a subclass of that class.  If it is not given
       any arguments, it simply returns $@.

       You should always make a copy of the exception object, rather than using $@ directly.
       This is necessary because if your "cleanup()" function uses "eval", or calls something
       which uses it, then $@ is overwritten.  Copying the exception preserves it for the call to
       "do_something_with_exception()".

       Exception objects also provide a caught method so you can write:

	 if ( my $e = My::Error->caught() ) {
	     cleanup();
	     do_something_with_exception($e);
	 }

   Uncatchable Exceptions
       Internally, the "caught()" method will call "isa()" on the exception object.  You could
       make an exception "uncatchable" by overriding "isa()" in that class like this:

	package Exception::Uncatchable;

	sub isa { shift->rethrow }

       Of course, this only works if you always call "Exception::Class->caught()" after an
       "eval".

USAGE RECOMMENDATION
       If you're creating a complex system that throws lots of different types of exceptions,
       consider putting all the exception declarations in one place.  For an app called Foo you
       might make a "Foo::Exceptions" module and use that in all your code.  This module could
       just contain the code to make "Exception::Class" do its automagic class creation.  Doing
       this allows you to more easily see what exceptions you have, and makes it easier to keep
       track of them.

       This might look something like this:

	 package Foo::Bar::Exceptions;

	 use Exception::Class (
	     Foo::Bar::Exception::Senses =>
		 { description => 'sense-related exception' },

	     Foo::Bar::Exception::Smell => {
		 isa	     => 'Foo::Bar::Exception::Senses',
		 fields      => 'odor',
		 description => 'stinky!'
	     },

	     Foo::Bar::Exception::Taste => {
		 isa	     => 'Foo::Bar::Exception::Senses',
		 fields      => [ 'taste', 'bitterness' ],
		 description => 'like, gag me with a spoon!'
	     },

	     ...
	 );

       You may want to create a real module to subclass Exception::Class::Base as well,
       particularly if you want your exceptions to have more methods.

   Subclassing Exception::Class::Base
       As part of your usage of "Exception::Class", you may want to create your own base
       exception class which subclasses Exception::Class::Base.  You should feel free to subclass
       any of the methods documented above.  For example, you may want to subclass "new()" to add
       additional information to your exception objects.

Exception::Class FUNCTIONS
       The "Exception::Class" method offers one function, "Classes()", which is not exported.
       This method returns a list of the classes that have been created by calling the
       "Exception::Class" import() method.  Note that this is all the subclasses that have been
       created, so it may include subclasses created by things like CPAN modules, etc.	Also note
       that if you simply define a subclass via the normal Perl method of setting @ISA or "use
       base", then your subclass will not be included.

SUPPORT
       Please submit bugs to the CPAN RT system at
       http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Exception%3A%3AClass or via email at
       bug-exception-class@rt.cpan.org.

DONATIONS
       If you'd like to thank me for the work I've done on this module, please consider making a
       "donation" to me via PayPal. I spend a lot of free time creating free software, and would
       appreciate any support you'd care to offer.

       Please note that I am not suggesting that you must do this in order for me to continue
       working on this particular software. I will continue to do so, inasmuch as I have in the
       past, for as long as it interests me.

       Similarly, a donation made in this way will probably not make me work on this software
       much more, unless I get so many donations that I can consider working on free software
       full time, which seems unlikely at best.

       To donate, log into PayPal and send money to autarch@urth.org or use the button on this
       page: <http://www.urth.org/~autarch/fs-donation.html>

AUTHOR
       Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE
       This software is copyright (c) 2013 by Dave Rolsky.

       This is free software; you can redistribute it and/or modify it under the same terms as
       the Perl 5 programming language system itself.

perl v5.16.3				    2013-02-24			      Exception::Class(3)
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