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autodie::exception(3perl) Perl Programmers Reference Guide autodie::exception(3perl)

NAME

autodie::exception - Exceptions from autodying functions.

SYNOPSIS

    eval {
        use autodie;

        open(my $fh, '<', 'some_file.txt');

        ...
    };

    if (my $E = $@) {
        say "Ooops!  ",$E->caller," had problems: $@";
    }

DESCRIPTION

When an autodie enabled function fails, it generates an "autodie::exception" object. This can be interrogated to determine further information about the error that occurred.

This document is broken into two sections; those methods that are most useful to the end-developer, and those methods for anyone wishing to subclass or get very familiar with "autodie::exception".

Common Methods

These methods are intended to be used in the everyday dealing of exceptions.

The following assume that the error has been copied into a separate scalar:

    if ($E = $@) {
        ...
    }

This is not required, but is recommended in case any code is called which may reset or alter $@.

args

    my $array_ref = $E->args;

Provides a reference to the arguments passed to the subroutine that died.

function

    my $sub = $E->function;

The subroutine (including package) that threw the exception.

file

    my $file = $E->file;

The file in which the error occurred (eg, "myscript.pl" or "MyTest.pm").

package

    my $package = $E->package;

The package from which the exceptional subroutine was called.

caller

    my $caller = $E->caller;

The subroutine that called the exceptional code.

line

    my $line = $E->line;

The line in "$E->file" where the exceptional code was called.

context

    my $context = $E->context;

The context in which the subroutine was called by autodie; usually the same as the context in which you called the autodying subroutine. This can be 'list', 'scalar', or undefined (unknown). It will never be 'void', as "autodie" always captures the return value in one way or another.

For some core functions that always return a scalar value regardless of their context (eg, "chown"), this may be 'scalar', even if you used a list context.

return

    my $return_value = $E->return;

The value(s) returned by the failed subroutine. When the subroutine was called in a list context, this will always be a reference to an array containing the results. When the subroutine was called in a scalar context, this will be the actual scalar returned.

errno

    my $errno = $E->errno;

The value of $! at the time when the exception occurred.

NOTE: This method will leave the main "autodie::exception" class and become part of a role in the future. You should only call "errno" for exceptions where $! would reasonably have been set on failure.

eval_error

    my $old_eval_error = $E->eval_error;

The contents of $@ immediately after autodie triggered an exception. This may be useful when dealing with modules such as Text::Balanced that set (but do not throw) $@ on error.

matches

    if ( $e->matches('open') ) { ... }

    if ( 'open' ~~ $e ) { ... }

"matches" is used to determine whether a given exception matches a particular role.

An exception is considered to match a string if:

  • For a string not starting with a colon, the string exactly matches the package and subroutine that threw the exception. For example, "MyModule::log". If the string does not contain a package name, "CORE::" is assumed.
  • For a string that does start with a colon, if the subroutine throwing the exception does that behaviour. For example, the "CORE::open" subroutine does ":file", ":io" and ":all".

    See "CATEGORIES" in autodie for further information.

    On Perl 5.10 and above, using smart-match ("~~") with an "autodie::exception" object will use "matches" underneath. This module used to recommend using smart-match with the exception object on the left hand side, but in future Perls that is likely to stop working. The smart-match facility of this class should only be used with the exception object on the right hand side. Having the exception object on the right is both future-proof and portable to older Perls, back to 5.10. Beware that this facility can only be relied upon when it is certain that the exception object actually is an "autodie::exception" object; it is no more capable than an explicit call to the "matches" method.

Advanced methods

The following methods, while usable from anywhere, are primarily intended for developers wishing to subclass "autodie::exception", write code that registers custom error messages, or otherwise work closely with the "autodie::exception" model.

register

    autodie::exception->register( 'CORE::open' => \&mysub );

The "register" method allows for the registration of a message handler for a given subroutine. The full subroutine name including the package should be used.

Registered message handlers will receive the "autodie::exception" object as the first parameter.

add_file_and_line

    say "Problem occurred",$@->add_file_and_line;

Returns the string " at %s line %d", where %s is replaced with the filename, and %d is replaced with the line number.

Primarily intended for use by format handlers.

stringify

    say "The error was: ",$@->stringify;

Formats the error as a human readable string. Usually there's no reason to call this directly, as it is used automatically if an "autodie::exception" object is ever used as a string.

Child classes can override this method to change how they're stringified.

format_default

    my $error_string = $E->format_default;

This produces the default error string for the given exception, without using any registered message handlers. It is primarily intended to be called from a message handler when they have been passed an exception they don't want to format.

Child classes can override this method to change how default messages are formatted.

new

    my $error = autodie::exception->new(
        args => \@_,
        function => "CORE::open",
        errno => $!,
        context => 'scalar',
        return => undef,
    );

Creates a new "autodie::exception" object. Normally called directly from an autodying function. The "function" argument is required, its the function we were trying to call that generated the exception. The "args" parameter is optional.

The "errno" value is optional. In versions of "autodie::exception" 1.99 and earlier the code would try to automatically use the current value of $!, but this was unreliable and is no longer supported.

Atrributes such as package, file, and caller are determined automatically, and cannot be specified.

SEE ALSO

autodie, autodie::exception::system

LICENSE

Copyright (C)2008 Paul Fenwick

This is free software. You may modify and/or redistribute this code under the same terms as Perl 5.10 itself, or, at your option, any later version of Perl 5.

AUTHOR

Paul Fenwick <pjf@perltraining.com.au>
2020-06-07 perl v5.30.3