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

version 1.45

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";

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.

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.

  Each field name must be a legal Perl identifier: it starts with a ASCII letter or underscore, and is followed by zero or more ASCII letters, ASCII digits, or underscores. If a field name does not match this, the creation of that exception class croaks.

  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.

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.

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);
}

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.

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.

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.

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.

Bugs may be submitted at https://github.com/houseabsolute/Exception-Class/issues.

I am also usually active on IRC as 'autarch' on irc://irc.perl.org.

The source code repository for Exception-Class can be found at https://github.com/houseabsolute/Exception-Class.

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 (let's all have a chuckle at that together).

To donate, log into PayPal and send money to [email protected], or use the button at https://www.urth.org/fs-donation.html.

Dave Rolsky [email protected]

• Alexander Batyrshin [email protected]
• brian d foy [email protected]
• Leon Timmermans [email protected]
• Ricardo Signes [email protected]

This software is copyright (c) 2021 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.

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