NAME

failures - Minimalist exception hierarchy generator

VERSION

version 0.004

SYNOPSIS

use failures qw/io::file io::network/;
use Try::Tiny;
use Safe::Isa; # for $_isa

try {
    process_file or
        failure::io::file->throw("oops, something bad happened: $!");
}
catch {
    if   ( $_->$_isa("failure::io::file") ) {
        ...
    }
    elsif( $_->$_isa("failure::io") ) {
        ...
    }
    elsif( $_->$_isa("failure") ) {
        ...
    }
    else {
        ...
    }
}

DESCRIPTION

This module lets you define an exception hierarchy quickly and simply.

Here were my design goals:

• minimalist interface

• 80% of features in 20% of lines of code

• depend only on core modules (nearly achieved)

• support hierarchical error types

• identify errors types by name (class) not by parsing strings

• leave (possibly expensive) trace decisions to the thrower

Currently, failures is implemented in under 70 lines of code.

Failure objects are implemented with Class::Tiny to allow easy subclassing (see custom::failures), but Class::Tiny only requires core modules, so other than that exception, the 'core only' goal is achieved.

USAGE

Defining failure categories

use failures qw/foo::bar foo::baz/;

This will define the following classes in the failure namespace:

• failure

• failure::foo

• failure::foo::bar

• failure::foo::baz

Subclasses inherit, so failure::foo::bar is-a failure::foo and failure::foo is-a failure.

Attributes

A failure class has three attributes: msg, payload, and trace. Their usage is described below. Accessors exist for all three.

Throwing failures

The throw method of a failure class takes a single, optional argument that modifies how failure objects are stringified.

If no argument is given, a default message is generated if the object is stringified:

say failure::foo::bar->throw;
# Caught failure::foo::bar

With a single, non-hash-reference argument, the argument is used for the msg attribute and is appended if the object is stringified.

say failure::foo::bar->throw("Ouch!");
# Caught failure::foo::bar: Ouch!

With a hash reference argument, the msg key provides the string to append to the default error. If you have extra data to attach to the exception, use the payload key:

failure::foo::bar->throw({
    msg     => "Ouch!",
    payload => $extra_data,
});

If an optional trace key is provided, it is appended if the object is stringified. To loosely emulate die and provide a simple filename and line number, use the failure->line_trace class method:

failure::foo::bar->throw({
    msg => "Ouch!",
    trace => failure->line_trace,
});

# Caught failure::foo::bar: Ouch!
#
# Failure caught at <FILENAME> line <NUMBER>

To provide a trace just like the Carp module (including respecting @CARP_NOT) use the croak_trace or confess_trace class methods:

failure::foo::bar->throw({
    msg => "Ouch!",
    trace => failure->croak_trace,
});

# Caught failure::foo::bar: Ouch!
#
# Failure caught at <CALLING-FILENAME> line <NUMBER>

failure::foo::bar->throw({
    msg => "Ouch!",
    trace => failure->confess_trace,
});

# Caught failure::foo::bar: Ouch!
#
# Failure caught at <FILENAME> line <NUMBER>
#   [confess stack trace continues]

You can provide a trace key with any object that overrides stringification, like Devel::StackTrace:

failure::foo::bar->throw({
    msg => "Ouch!",
    trace => Devel::StackTrace->new,
});

# Caught failure::foo::bar: Ouch!
#
# [stringified Devel::StackTrace object]

Catching failures

Use Try::Tiny, of course. Within a catch block, you know that $_ is defined, but it still might be an unblessed reference or something that is risky to call isa on. If you load Safe::Isa, you get a code reference in $_isa that calls isa only on objects.

So catching looks like this:

use Try::Tiny;
use Safe::Isa;

try { ... }
catch {
    if ( $_->$_isa("failure::foo") ) {
        # handle it
    }
};

If you need to rethrow the exception, just use die:

elsif ( $_->$_isa("failure") ) {
    die $_;
}

Overriding failure class behavior

See custom::failures.

SEE ALSO

There are many error/exception systems on CPAN. This one is designed to be minimalist.

If you have more complex or substantial needs, people I know and trust seem to be recommending:

• Throwable — exceptions as a Moo/Moose role

• Throwable::X — Throwable extended with extra goodies

Here are other modules I found that weren't appropriate for my needs or didn't suit my taste:

• Class::Throwable — no hierarchy and always builds a full stack trace

• Error::Tiny — blends Try::Tiny and a trivial exception base class

• Exception::Base — complexity on par with Exception::Class, but highly optimized for speed

• Exception::Class — once highly recommended, but even the author now suggests Throwable

• Exception::Simple — very simple, but always uses caller and has no hierarchy

• Exception::Tiny — not bad, but always uses caller and setting up a hierarchy requires extra work

• Ouch — simple, well-thought out, but no hierarchy; also cutesy function names

Here are some that I'm very dubious about:

• Err — alpha since 2012

• Error — no longer recommended by maintainer

• errors — "still under design" since 2009

• Exception — dates back to 1996 and undocumented

SUPPORT

Bugs / Feature Requests

Please report any bugs or feature requests through the issue tracker at https://github.com/dagolden/failures/issues. You will be notified automatically of any progress on your issue.

Source Code

This is open source software. The code repository is available for public review and contribution under the terms of the license.

https://github.com/dagolden/failures

git clone https://github.com/dagolden/failures.git

AUTHOR

David Golden <dagolden@cpan.org>

CONTRIBUTOR

Michael Jemmeson <mjemmeson@cpan.org>

COPYRIGHT AND LICENSE

This software is Copyright (c) 2013 by David Golden.

This is free software, licensed under:

The Apache License, Version 2.0, January 2004

cpanm failure

perl -MCPAN -e shell
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