/ 
Error-errors-0.13
River stage zero No dependents
/ errors

NAME

errors - Error Handling for Perl

error-errors-pm error-errors-pm

STATUS

this module is still under design. Don't use it in production yet. See errors::Design for more information.

A few things in this documentation are not yet implemented.

NOTE: If you have suggestions as to how this module should behave, now is a great time to contact the author.

SYNOPSIS

use strict;
use warnings;
use errors;

use errors -class => 'UncoolError';

try {
    $cool = something();
    throw UncoolError("Something is not cool")
        if not $cool;
    assert($ok, "Everything is ok");
}
catch UncoolError with {
    my $e = shift;
    warn "$e";
}
catch UserError, RuntimeError with {
    # catch UserError or RuntimeError
    # $_ is the same as $_[0]
    warn;
}
except {
    # $@ is the same as $_[0]
    warn "Some other error: $@";
}
otherwise {
    warn "No error occurred in the try clause."
}
finally {
    cleanup();
};

DESCRIPTION

The errors module adds clean, simple, sane, flexible and usable error handling to Perl. The module does several things:

First, errors exports an error handling syntax that is very similar to Error.pm, but with a few improvements. (See [COMPARISON TO Error.pm])

Second, all errors that are thrown are first class Perl objects. They all inherit from the Exception class, which is provided by default. This allows you to manipulate errors in a consistent and intuitive way.

Third, The errors module makes it trivial to define your own error classes, and encourages you to do so. Simply define a class that inherits from Exception (or one of its subclasses).

Fourth, errors turns plain (string based) system errors and other die/croak errors into specific Perl objects. It uses heuristics on the error string to determine which Error class to use, and defaults to the RuntimeError class.

Fifth, errors provides dozens of predefined error classes that you can use or that get used automatically by the auto-objectification. These classes are in an inheritance hierarchy that should become standard for Perl.

Lastly, errors is designed to play nice with the modern Perl frameworks (like Moose) and the other popular error handling modules.

SIMPLE TO USE

The main goal of errors is to encourage the widespread use of error handling in Perl. In other languages like Python, coining your own named error classes and using raiseexcept is as common as using ifelse statements. Here's a Python example.

class FooError(Exception):
    pass

try:
    something()
except FooError as e:
    handle_error(e)

Now you can do that in Perl:

use errors;

package FooError;
use base 'Exception';
package MyModule;

try {
    something();
}
catch FooError with {
    my $e = shift;
    handle_error($e);
};

As you can see, using errors is simple and unobtrusive. Why not start all your programs with:

use strict;
use errors;
use warnings;

Defining your own error classes is also trivial, and errors provides an even more concise way to do it:

use errors -class => 'FooError';

In the catch/with clause, you can also use $@ (or $_) to access the current error object like this:

catch FooError with {
    handle_error($@);
};

USAGE

there are a few different usages of errors that you should be aware of:

use errors;

This exports the errors syntax, and loads all the errors functionality.

use errors -class => 'ClassName' [, -isa => 'BaseClass'];

The -class directive gives you a way to define an error subclass at compile time, in one simple line of code. You can optionally specify the base class. The default base class is Exception.

NOTE: This usage does not export the errors (try/catch) syntax.

use errors -with_using;

Unfortunately Moose and errors both export the with subroutine. If errors sees that Moose (or someone else) has already exported with, it will export the using subroutine instead:

use Moose;
use errors;
try {...} catch Exception using {...};

The -with_using directive tells errors to do this regardless.

SYNTAX

The errors module introduces a number of keyword constructs that you can use to create and manage error objects.

try { ... }

Like an eval block. After the code is evaluated, the appropriate clauses (catch, except, otherwise, finally) are called.

catch <error-selector> with { ... }

This clause is invoked when an error happens in the try block, and the class of the error object satisfies the ErrorSelector specified. You may specify many catch clauses, to deal with different error situations.

The <error-selector> can be any of the following forms:

# Class matches a specific error class
catch ErrorClass with { ... }
# Class matches a specific regexp
catch qr/.../ with { ... }
# A subroutine returns a true value
catch sub { ... } with { ... }
# One of a list of error selectors
catch selector1, selector2, selector3 with { ... }
# All of an array list of selectors
catch [ selector1, selector2, selector3 ] with { ... }
except { ... }

This clause is invoked when there is an error from the try block, but no catch clauses were invoked.

otherwise { ... }

This clause is invoked if there was no error in the try clause.

finally { ... }

This clause is always invoked as the final step in the try sequence, regardless of what happens.

throw("...");

The throw keyword is not actually exported. It is a method call on the Exception object. So you can use it indirectly or directly. These two calls are identical:

throw MyError("Something is wrong");
MyError->throw("Something is wrong");

You can also use throw to reraise an error in a catch/except block, like this:

$@->throw();
assert($value, "assertion message");

This function will throw AssertionError($message) error unless $value` is true.

ERROR OBJECTS

All errors are Perl objects. They all have the 'Exception' class as their topmost parent class. They all have the following methods and properties:

throw Exception($msg [, %properties]);

This method throws a new instance of the Exception class. It is described more fully above.

$@->text()

The text method gets or sets the error message for the object.

Stringification

All Exception objects turn into their text string value when used in string context.

Numification

All Exception objects turn into a unique number when used in numeric context.

PREDEFINED CLASSES

The errors module defines a number of error classes that it uses to cast errors into. You can also create error objects yourself using these classes. The classes are defined in a hierarchy:

+ Exception
  + StandardError
    + ArithmeticError
      + DivideByZeroError
    + AssertionError
    + IOError
      + IOFileError
        + IOFileOpenError
    + NotImplementedError
    + SyntaxError
    + RuntimeError
  + UserError
    + user defined errors should inherit from this

Some of these are obvious. Some deserve elaboration.

AssertionError

Indicates a failed assert call.

SyntaxError

Indicates a bad string eval.

NotImplementedError

You can throw this in a stub subroutine.

RuntimeError

Indicates an unknown error probably caused by a die statement..

NOTE: These error classes are still being determined. This list is not yet complete. The current hierarchy was influenced from these sources:

COMPARISON TO ERROR.PM

The trycatchthrow interface of both Errors.pm and errors.pm is very similar. You can use both in the same runtime process (but you can only use one or the other in the same class/package).

The errors module differs from the <Error> module in the following ways:

catch Selector with { ... }

The Selector for the catch clause can only be a single class name in Error.pm. In errors it is much more flexible. See documentation for details.

except { ... }

The except clause in Error.pm has weird semantics. In errors it just gets called if there is an error and no catch clause matches.

otherwise { ... }

The otherwise clause in Error.pm gets called if no other handler is appropriate. In errors, it behaves like an 'else' block. It is called when there is no error at all in the try clause.

Base Class

Errors in the Error module have a common base class of 'Error'. In errors, the base class is called 'Exception'.

FAQ

Q: What is the difference between 'errors' and 'exceptions'?

A: Four letters.

Q: Is errors performant?

A: Yes. Very. The module is small, simple, has no dependencies and no string evals.

Q: Why another error/exception module?

A: Because it has the perfect name.

ACKNOWLEDGEMENTS

The original code and ideas for this module were taken from Error.pm.

AUTHOR

ingy döt Net <ingy@cpan.org>

COPYRIGHT

copyright 2009-2014. Ingy döt Net.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

See http://www.perl.com/perl/misc/Artistic.html