autodi...eption(3perl) Perl Programmers Reference Guide autodi...eption(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>
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