Moose::Manual::Exceptions(3) User Contributed Perl Documentation Moose::Manual::Exceptions(3)
NAME
Moose::Manual::Exceptions - Moose's exceptions
VERSION
version 2.1202
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
";
}
}
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
";
} 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
";
}
}
}
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 $line
";
} else {
# you've got some other kind of exception, so just print it
# note: all Moose::Exception objects will stringify to a useful error message
warn "$exception
";
}
};
Moose Exception Types
These are documented in Moose::Manual::Exceptions::Manifest.
AUTHORS
o Stevan Little <stevan.little@iinteractive.com>
o Dave Rolsky <autarch@urth.org>
o Jesse Luehrs <doy@tozt.net>
o Shawn M Moore <code@sartak.org>
o XXXX XXX'XX (Yuval Kogman) <nothingmuch@woobling.org>
o Karen Etheridge <ether@cpan.org>
o Florian Ragwitz <rafl@debian.org>
o Hans Dieter Pearcey <hdp@weftsoar.net>
o Chris Prather <chris@prather.org>
o Matt S Trout <mst@shadowcat.co.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 v5.18.2 2014-01-19 Moose::Manual::Exceptions(3)