NAME¶
Exception::Class - A module that allows you to declare real exception classes in
Perl
VERSION¶
version 1.38
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:
- •
- 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.
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) 2014 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.