1Exception::Class(3) User Contributed Perl Documentation Exception::Class(3)
2
6 Exception::Class - A module that allows you to declare real exception
7 classes in Perl
8
10 version 1.44
11
13 use Exception::Class (
14 'MyException',
15 'AnotherException' => { isa => 'MyException' },
17 'YetAnotherException' => {
19 isa => 'AnotherException',
20 description => 'These exceptions are related to IPC'
21 },
22 'ExceptionWithFields' => {
24 isa => 'YetAnotherException',
25 fields => [ 'grandiosity', 'quixotic' ],
26 alias => 'throw_fields',
27 },
28 );
29 use Scalar::Util qw( blessed );
30 use Try::Tiny;
31 try {
33 MyException->throw( error => 'I feel funny.' );
34 }
35 catch {
36 die $_ unless blessed $_ && $_->can('rethrow');
37 if ( $_->isa('Exception::Class') ) {
39 warn $_->error, "\n", $_->trace->as_string, "\n";
40 warn join ' ', $_->euid, $_->egid, $_->uid, $_->gid, $_->pid, $_->time;
41 exit;
43 }
44 elsif ( $_->isa('ExceptionWithFields') ) {
45 if ( $_->quixotic ) {
46 handle_quixotic_exception();
47 }
48 else {
49 handle_non_quixotic_exception();
50 }
51 }
52 else {
53 $_->rethrow;
54 }
55 };
56 # without Try::Tiny
58 eval { ... };
59 if ( my $e = Exception::Class->caught ) { ... }
60 # use an alias - without parens subroutine name is checked at
62 # compile time
63 throw_fields error => "No strawberry", grandiosity => "quite a bit";
64
66 RECOMMENDATION 1: If you are writing modern Perl code with Moose or Moo
67 I highly recommend using Throwable instead of this module.
68 RECOMMENDATION 2: Whether or not you use Throwable, you should use
70 Try::Tiny.
71 Exception::Class allows you to declare exception hierarchies in your
73 modules in a "Java-esque" manner.
74 It features a simple interface allowing programmers to 'declare'
76 exception classes at compile time. It also has a base exception class,
77 Exception::Class::Base, that can be easily extended.
78 It is designed to make structured exception handling simpler and better
80 by encouraging people to use hierarchies of exceptions in their
81 applications, as opposed to a single catch-all exception class.
82 This module does not implement any try/catch syntax. Please see the
84 "OTHER EXCEPTION MODULES (try/catch syntax)" section for more
85 information on how to get this syntax.
86 You will also want to look at the documentation for
88 Exception::Class::Base, which is the default base class for all
89 exception objects created by this module.
90
92 Importing "Exception::Class" allows you to automagically create
93 Exception::Class::Base subclasses. You can also create subclasses via
94 the traditional means of defining your own subclass with @ISA. These
95 two methods may be easily combined, so that you could subclass an
96 exception class defined via the automagic import, if you desired this.
97 The syntax for the magic declarations is as follows:
99 'MANDATORY CLASS NAME' => \%optional_hashref
101 The hashref may contain the following options:
103 · isa
105 This is the class's parent class. If this isn't provided then the
107 class name in $Exception::Class::BASE_EXC_CLASS is assumed to be
108 the parent (see below).
109 This parameter lets you create arbitrarily deep class hierarchies.
111 This can be any other Exception::Class::Base subclass in your
112 declaration or a subclass loaded from a module.
113 To change the default exception class you will need to change the
115 value of $Exception::Class::BASE_EXC_CLASS before calling "import".
116 To do this simply do something like this:
117 BEGIN { $Exception::Class::BASE_EXC_CLASS = 'SomeExceptionClass'; }
119 If anyone can come up with a more elegant way to do this please let
121 me know.
122 CAVEAT: If you want to automagically subclass an
124 Exception::Class::Base subclass loaded from a file, then you must
125 compile the class (via use or require or some other magic) before
126 you import "Exception::Class" or you'll get a compile time error.
127 · fields
129 This allows you to define additional attributes for your exception
131 class. Any field you define can be passed to the "throw" or "new"
132 methods as additional parameters for the constructor. In addition,
133 your exception object will have an accessor method for the fields
134 you define.
135 This parameter can be either a scalar (for a single field) or an
137 array reference if you need to define multiple fields.
138 Fields will be inherited by subclasses.
140 · alias
142 Specifying an alias causes this class to create a subroutine of the
144 specified name in the caller's namespace. Calling this subroutine
145 is equivalent to calling "<class>->throw(@_)" for the given
146 exception class.
147 Besides convenience, using aliases also allows for additional
149 compile time checking. If the alias is called without parentheses,
150 as in "throw_fields "an error occurred"", then Perl checks for the
151 existence of the "throw_fields" subroutine at compile time. If
152 instead you do "ExceptionWithFields->throw(...)", then Perl checks
153 the class name at runtime, meaning that typos may sneak through.
154 · description
156 Each exception class has a description method that returns a fixed
158 string. This should describe the exception class (as opposed to any
159 particular exception object). This may be useful for debugging if
160 you start catching exceptions you weren't expecting (particularly
161 if someone forgot to document them) and you don't understand the
162 error messages.
163 The "Exception::Class" magic attempts to detect circular class
165 hierarchies and will die if it finds one. It also detects missing links
166 in a chain, for example if you declare Bar to be a subclass of Foo and
167 never declare Foo.
168
170 If you are interested in adding try/catch/finally syntactic sugar to
171 your code then I recommend you check out Try::Tiny. This is a great
172 module that helps you ignore some of the weirdness with "eval" and $@.
173 Here's an example of how the two modules work together:
174 use Exception::Class ( 'My::Exception' );
176 use Scalar::Util qw( blessed );
177 use Try::Tiny;
178 try {
180 might_throw();
181 }
182 catch {
183 if ( blessed $_ && $_->isa('My::Exception') ) {
184 handle_it();
185 }
186 else {
187 die $_;
188 }
189 };
190 Note that you cannot use "Exception::Class->caught" with Try::Tiny.
192
194 "Exception::Class" provides some syntactic sugar for catching
195 exceptions in a safe manner:
196 eval {...};
198 if ( my $e = Exception::Class->caught('My::Error') ) {
200 cleanup();
201 do_something_with_exception($e);
202 }
203 The "caught" method takes a class name and returns an exception object
205 if the last thrown exception is of the given class, or a subclass of
206 that class. If it is not given any arguments, it simply returns $@.
207 You should always make a copy of the exception object, rather than
209 using $@ directly. This is necessary because if your "cleanup" function
210 uses "eval", or calls something which uses it, then $@ is overwritten.
211 Copying the exception preserves it for the call to
212 "do_something_with_exception".
213 Exception objects also provide a caught method so you can write:
215 if ( my $e = My::Error->caught ) {
217 cleanup();
218 do_something_with_exception($e);
219 }
220 Uncatchable Exceptions
222 Internally, the "caught" method will call "isa" on the exception
223 object. You could make an exception "uncatchable" by overriding "isa"
224 in that class like this:
225 package Exception::Uncatchable;
227 sub isa { shift->rethrow }
229 Of course, this only works if you always call
231 "Exception::Class->caught" after an "eval".
232
234 If you're creating a complex system that throws lots of different types
235 of exceptions, consider putting all the exception declarations in one
236 place. For an app called Foo you might make a "Foo::Exceptions" module
237 and use that in all your code. This module could just contain the code
238 to make "Exception::Class" do its automagic class creation. Doing this
239 allows you to more easily see what exceptions you have, and makes it
240 easier to keep track of them.
241 This might look something like this:
243 package Foo::Bar::Exceptions;
245 use Exception::Class (
247 Foo::Bar::Exception::Senses =>
248 { description => 'sense-related exception' },
249 Foo::Bar::Exception::Smell => {
251 isa => 'Foo::Bar::Exception::Senses',
252 fields => 'odor',
253 description => 'stinky!'
254 },
255 Foo::Bar::Exception::Taste => {
257 isa => 'Foo::Bar::Exception::Senses',
258 fields => [ 'taste', 'bitterness' ],
259 description => 'like, gag me with a spoon!'
260 },
261 ...
263 );
264 You may want to create a real module to subclass Exception::Class::Base
266 as well, particularly if you want your exceptions to have more methods.
267 Subclassing Exception::Class::Base
269 As part of your usage of "Exception::Class", you may want to create
270 your own base exception class which subclasses Exception::Class::Base.
271 You should feel free to subclass any of the methods documented above.
272 For example, you may want to subclass "new" to add additional
273 information to your exception objects.
274
276 The "Exception::Class" method offers one function, "Classes", which is
277 not exported. This method returns a list of the classes that have been
278 created by calling the "Exception::Class" "import" method. Note that
279 this is all the subclasses that have been created, so it may include
280 subclasses created by things like CPAN modules, etc. Also note that if
281 you simply define a subclass via the normal Perl method of setting @ISA
282 or "use base", then your subclass will not be included.
283
285 Bugs may be submitted at
286 <https://github.com/houseabsolute/Exception-Class/issues>.
287 I am also usually active on IRC as 'autarch' on "irc://irc.perl.org".
289
291 The source code repository for Exception-Class can be found at
292 <https://github.com/houseabsolute/Exception-Class>.
293
295 If you'd like to thank me for the work I've done on this module, please
296 consider making a "donation" to me via PayPal. I spend a lot of free
297 time creating free software, and would appreciate any support you'd
298 care to offer.
299 Please note that I am not suggesting that you must do this in order for
301 me to continue working on this particular software. I will continue to
302 do so, inasmuch as I have in the past, for as long as it interests me.
303 Similarly, a donation made in this way will probably not make me work
305 on this software much more, unless I get so many donations that I can
306 consider working on free software full time (let's all have a chuckle
307 at that together).
308 To donate, log into PayPal and send money to autarch@urth.org, or use
310 the button at <http://www.urth.org/~autarch/fs-donation.html>.
311
313 Dave Rolsky <autarch@urth.org>
314
316 · Alexander Batyrshin <0x62ash@gmail.com>
317 · Leon Timmermans <fawaka@gmail.com>
319 · Ricardo Signes <rjbs@cpan.org>
321
323 This software is copyright (c) 2017 by Dave Rolsky.
324 This is free software; you can redistribute it and/or modify it under
326 the same terms as the Perl 5 programming language system itself.
327 The full text of the license can be found in the LICENSE file included
329 with this distribution.
330perl v5.28.1 2017-12-10 Exception::Class(3)