Moose::Manual::Exceptions(3pm) User Contributed Perl Documentation Moose::Manual::Exceptions(3pm)



NAME
       Moose::Manual::Exceptions - Moose's exceptions

VERSION
       version 2.1213

Exceptions in Moose
       Moose will throw an instance of "Moose::Exception" when it encounters an error condition.
       There are many specific subclasses of Moose::Exception, each designed specifically for its
       particular error condition. These subclasses have attributes that contain relevant
       information, such as a stack trace, related metaclass objects, etc.

Handling Moose Exceptions
       Because Moose's exceptions use the standard "die" mechanism, you are free to catch and
       handle errors however you like. You could use Perl's builtin "eval" to catch Moose
       exceptions.  However due to the subtle problems "eval" can introduce into your programs,
       the Moose team strongly recommends using Try::Tiny instead. Please refer to Try::Tiny's
       documentation for a discussion of how "eval" is dangerous.

       The following example demonstrates how to catch and inspect a Moose::Exception. For the
       sake of simplicity, we will cause a very simple error. The "extends" keywords expects a
       list of superclass names. If we pass no superclass names, Moose will throw an instance of
       Moose::Exception::ExtendsMissingArgs.

   Catching with Try::Tiny
           use warnings;
           use strict;
           use Try::Tiny;

           try {
               package Example::Exception;
               use Moose;
               extends; # <-- error!
           } catch {
               # $_ contains the instance of the exception thrown by the above try block
               # $_ may get clobbered, so we should copy its value to another variable
               my $exception = $_;

               # exception objects are not ubiquitous in Perl, so we must check whether $exception is blessed
               # we also need to ensure that $exception is actually the kind of exception we were expecting
               if ( blessed $exception && $exception->isa("Moose::Exception::ExtendsMissingArgs") ) {
                   # fetch attributes from the $exception object and display a friendly error to the user
                   my $class_name = $exception->class_name;
                   warn "You forgot to specify the superclass of $class_name, dummy!";
               } else {
                   # you've got some other kind of exception, so just print it
                   # note: all Moose::Exception objects will stringify to a useful error message
                   warn "$exception\n";
               }
           }

   Example of catching ValidationFailedForTypeConstraint
           use warnings;
           use strict;
           use Try::Tiny;

           {
               package Person;
               use Moose;
               use Moose::Util::TypeConstraints;

               subtype 'NameStr',
               as 'Str',
               where { $_ =~ /^[a-zA-Z]+$/; };

               has 'age' => (
                   is       => 'ro',
                   isa      => 'Int',
                   required => 1
               );

               has 'name' => (
                   is       => 'ro',
                   isa      => 'NameStr',
                   required => 1
               );
           }

           my $person;
           while( !$person ) {
               try {
                   print "Enter your age : ";
                   my $age = <STDIN>;
                   chomp $age;
                   print "Enter your name : ";
                   my $name = <STDIN>;
                   chomp $name;
                   $person = Person->new( age  => $age,
                                          name => $name
                                        );
                   my $person_name = $person->name;
                   my $person_age  = $person->age;
                   print "$person_name is $person_age years old\n";
               } catch {
                   my $exception = $_;

                   if ( blessed $exception && $exception->isa("Moose::Exception::ValidationFailedForTypeConstraint") ) {

                       # fetch attributes from the $exception object and display a friendly error to the user
                       my $attribute_name = $exception->attribute->name;
                       my $type_name = $exception->type->name;
                       my $value = $exception->value;

                       warn "You entered $value for $attribute_name, which is not $type_name!";
                   } else {

                       # you've got some other kind of exception, so just print it
                       # note: all Moose::Exception objects will stringify to a useful error message
                       warn "$exception\n";
                   }
               }
           }

   Example of catching AttributeIsRequired
           use warnings;
           use strict;
           use Try::Tiny;

           {
               package Example::RequiredAttribute;
               use Moose;

               has 'required_attribute' => (
                   is       => 'ro',
                   isa      => 'Int',
                   required => 1
               );
           }

           try {
               # we're not passing required_attribute, so it'll throw an exception
               my $object = Example::RequiredAttribute->new();
           } catch {
               my $exception = $_;
               if ( blessed $exception && $exception->isa("Moose::Exception::AttributeIsRequired") ) {

                   # fetch attributes from the $exception object and display only
                   # the topmost frame of the stack trace
                   my $attribute_name = $exception->attribute->name;
                   my $trace = $exception->trace;

                   my $frame = $trace->frame(0);

                   my $message = $exception->message;
                   my $file    = $frame->{filename};
                   my $line    = $frame->{line};

                   warn "$message at $file $line\n";
               } else {

                   # you've got some other kind of exception, so just print it
                   # note: all Moose::Exception objects will stringify to a useful error message
                   warn "$exception\n";
               }
           };

Moose Exception Types
       These are documented in Moose::Manual::Exceptions::Manifest.

AUTHORS
       ·   Stevan Little <stevan.little AT iinteractive.com>

       ·   Dave Rolsky <autarch AT urth.org>

       ·   Jesse Luehrs <doy AT tozt.net>

       ·   Shawn M Moore <code AT sartak.org>

       ·   XXXX XXX'XX (Yuval Kogman) <nothingmuch AT woobling.org>

       ·   Karen Etheridge <ether AT cpan.org>

       ·   Florian Ragwitz <rafl AT debian.org>

       ·   Hans Dieter Pearcey <hdp AT weftsoar.net>

       ·   Chris Prather <chris AT prather.org>

       ·   Matt S Trout <mst AT shadowcat.uk>

COPYRIGHT AND LICENSE
       This software is copyright (c) 2006 by Infinity Interactive, Inc..

       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 v5.20.1                                2014-09-25             Moose::Manual::Exceptions(3pm)