Felipe Gasper
and 1 contributors

NAME

X::Tiny - Base class for a bare-bones exception factory

SYNOPSIS

    package My::Module::X;

    use parent qw( X::Tiny );

    #----------------------------------------------------------------------

    package My::Module::X::Base;

    use parent qw( X::Tiny::Base );

    #----------------------------------------------------------------------

    package My::Module::X::IO;

    use parent qw( My::Module::X::Base );

    #----------------------------------------------------------------------

    package My::Module::X::Blah;

    use parent qw( My::Module::X::Base );

    sub _new {
        my ($class, @args) = @_;

        my $self = $class->SUPER::_new('Blah blah', @args);

        return bless $self, $class;
    }

    #----------------------------------------------------------------------

    package main;

    local $@;   #always!
    eval {
        die My::Module::X->create('IO', 'The message', key1 => val1, … );
    };

    if ( my $err = $@ ) {
        print $err->get('key1');
    }

    die My::Module::X->create('Blah', key1 => val1, … );

DESCRIPTION

This stripped-down exception framework provides a baseline of functionality for distributions that want to expose exception hierarchies with minimal fuss. It’s a pattern that I implemented in some other distributions I created and didn’t want to copy/paste around.

BENEFITS OF EXCEPTIONS

Exceptions are better for error reporting in Perl than the C-style “return in failure” pattern. In brief, you should use exceptions because they are a logical, natural way to report failures: if you’re given a set of instructions, and something goes wrong in one of those instructions, it makes sense to stop and go back to see what to do in response to the problem.

Perl’s built-ins unwisely make the caller responsibile for error checking—as a result of which much Perl code fails to check for failures from those built-ins, which makes for far more difficult debugging when some code down the line just mysteriously produces an unexpected result. The more sensible pattern is for an exception to be thrown at the spot where the error occurred.

Perl’s default exceptions are just scalars. A more useful pattern is to throw exception objects whose type and attributes can facilitate meaningful error checking; for example, you may not care if a call to unlink() fails with ENOENT, so you can just ignore that failure. Or, you might care, but you might prefer just to warn() rather than to stop what you’re doing.

X::Tiny is one of many CPAN modules that facilitates this pattern. What separates X::Tiny from other such modules is its light weight: the only “heavy” dependency is overload, which is (in my experience) a reasonable trade-off for the helpfulness of having stack traces on uncaught exceptions. (The stack trace is custom logic, much lighter than Carp.)

FEATURES

  • Super-lightweight: No exceptions are loaded until they’re needed.

  • Simple, flexible API

  • String overload with stack trace

  • Minimal code necessary

USAGE

You’ll first create a factory class that subclasses X::Tiny. (In the SYNOPSIS’s example, this module is My::Module::X.) All of your exceptions must exist under that factory class’s namespace.

You’ll then create a base exception class for your distribution. In the SYNOPSIS’s example, this module is My::Module::X::Base. Your distribution’s other exceptions should all subclass this one.

METHODS

There’s only one method in the factory class:

CLASS->create( TYPE, ARG1, ARG2, .. )

To create an exception, call the create() method of your factory class. This will load the exception class if it’s not already in memory. The TYPE you pass in is equivalent to the exception class’s module name but with the factory class’s name chopped off the left part. So, if you call:

    My::Module::X->create('BadInput', 'foo', 'bar')

… this will instantiate and return an instance of My::Module::X::BadInput, with the arguments foo and bar.

EXCEPTION OBJECTS

See X::Tiny::Base for more information about the features that that module exposes to subclasses.

DESIGN CONSIDERATIONS

Admittedly, the lazy-loading behavior here embodies a generally-unwise practice of doing failure-prone work (i.e., loading a module at runtime) in the process of reporting a failure. In my own experience, though, that’s a reasonable tradeoff for the expressiveness of typed exceptions.

Do be sure that any failure-prone work you do as part of exception instantiation has its own failure-checking mechanism. There really are not meant to be “sub-failures” here!

REPOSITORY

https://github.com/FGasper/p5-X-Tiny

AUTHOR

Felipe Gasper (FELIPE)

COPYRIGHT

Copyright 2017 by Gasper Software Consulting, LLC

LICENSE

This distribution is released under the same license as Perl.