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

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',
          },
      );

      # try
      eval { MyException->throw( error => 'I feel funny.' ) };

      my $e;

      # catch
      if ( $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;
      }

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

DESCRIPTION
    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:

    * 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.

    * 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.

    * 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.

    * 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.

Catching Exceptions
    `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.

OTHER EXCEPTION MODULES (try/catch syntax)
    If you are interested in adding try/catch/finally syntactic sugar to
    your code then I recommend you check out U. Arun Kumar's `Error.pm'
    module, which implements this syntax. It also includes its own base
    exception class, `Error::Simple'.

    If you would prefer to use the Exception::Class::Base class included
    with this module, you'll have to add this to your code somewhere:

      push @Exception::Class::Base::ISA, 'Error'
          unless Exception::Class::Base->isa('Error');

    It's a hack but apparently it works.

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
    Copyright (c) 2000-2009 David Rolsky. All rights reserved. This program
    is free software; you can redistribute it and/or modify it under the
    same terms as Perl itself.

    The full text of the license can be found in the LICENSE file included
    with this module.

