failures(3) 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.

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 <[email protected]>

CONTRIBUTOR

Michael Jemmeson <[email protected]>

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

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