NAME

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

VERSION

version 2.1301

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@iinteractive.com>

• Dave Rolsky <autarch@urth.org>

• Jesse Luehrs <doy@tozt.net>

• Shawn M Moore <code@sartak.org>

• יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>

• Karen Etheridge <ether@cpan.org>

• Florian Ragwitz <rafl@debian.org>

• Hans Dieter Pearcey <hdp@weftsoar.net>

• Chris Prather <chris@prather.org>

• 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.