Moose::Manual::Exceptions

Section: User Contributed Perl Documentation (3)
Updated: 2018-05-16
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NAME

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

VERSION

version 2.2011  

EXCEPTIONS IN MOOSE

Moose will throw an exception for all error conditions. This applies both to code in the Moose core as well as to all code generated when a class is made immutable. All exceptions are subclasses of the "Moose::Exception" class.

Each type of error has its own unique subclass, and many subclasses have additional attributes to provide more information about the error's context, such as what classes or roles were involved.  

EXCEPTION STRINGIFICATION

By default, Moose exceptions remove Moose internals from the stack trace. If you set the "MOOSE_FULL_EXCEPTION" environment variable to a true value, then the Moose internals will be included in the trace.  

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 an "eval" block to catch Moose exceptions. However, 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, but $_ may get clobbered, so we should copy its value to
        # another variable.
        my $e = $_;

        # Exception objects are not ubiquitous in Perl, so we must check
        # whether $e is blessed. We also need to ensure that $e is actually
        # the kind of exception we were expecting.
        if ( blessed $e
            && $e->isa('Moose::Exception::ExtendsMissingArgs') ) {

            my $class_name = $e->class_name;
            warn "You forgot to specify a superclass for $class_name, silly!";
        }

        # It's either another type of an object or not an object at all.
        else {
            warn "$e\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 $e = $_;

            if (
                blessed $e
                && $e->isa(
                    'Moose::Exception::ValidationFailedForTypeConstraint')
                ) {

                my $attribute_name = $e->attribute->name;
                my $type_name      = $e->type->name;
                my $value          = $e->value;

                warn
                    "You entered $value for $attribute_name, which is not a $type_name!";
            }
            else {
                warn "$e\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 $e = $_;
        if ( blessed $e && $e->isa('Moose::Exception::AttributeIsRequired') )
        {
            warn $e->message, "\n";
        }
        else {
            warn "$e\n";
        }
    };

 

MOOSE EXCEPTION CLASSES

All the exception classes are listed 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.


 

Index

NAME
VERSION
EXCEPTIONS IN MOOSE
EXCEPTION STRINGIFICATION
HANDLING MOOSE EXCEPTIONS
Catching with Try::Tiny
Example of catching ValidationFailedForTypeConstraint
Example of catching AttributeIsRequired
MOOSE EXCEPTION CLASSES
AUTHORS
COPYRIGHT AND LICENSE
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