Error(3pm)
NAME
Error - Error/exception handling in an OO-ish way
WARNING
Using the "Error" module is no longer recommended due to the black- magical nature of its syntactic sugar, which often tends to break. Its maintainers have stopped actively writing code that uses it, and discourage people from doing so. See the "SEE ALSO" section below for better recommendations.
SYNOPSIS
use Error qw(:try);
throw Error::Simple( "A simple error");
sub xyz {
...
record Error::Simple("A simple error")
and return;
}
unlink($file) or throw Error::Simple("$file: $!",$!);
try {
do_some_stuff();
die "error!" if $condition;
throw Error::Simple "Oops!" if $other_condition;
}
catch Error::IO with {
my $E = shift;
print STDERR "File ", $E->{'-file'}, " had a problem\n";
}
except {
my $E = shift;
my $general_handler=sub {send_message $E->{-description}};
return {
UserException1 => $general_handler,
UserException2 => $general_handler
};
}
otherwise {
print STDERR "Well I don't know what to say\n";
}
finally {
close_the_garage_door_already(); # Should be reliable
}; # Don't forget the trailing ; or you might be surprised
DESCRIPTION
The "Error" package provides two interfaces. Firstly "Error" provides a procedural interface to exception handling. Secondly "Error" is a base class for errors/exceptions that can either be thrown, for subsequent catch, or can simply be recorded. Errors in the class "Error" should not be thrown directly, but the user should throw errors from a sub-class of "Error".
PROCEDURAL INTERFACE
"Error" exports subroutines to perform exception handling. These will
be exported if the ":try" tag is used in the "use" line.
try BLOCK CLAUSES
"try" is the main subroutine called by the user. All other
subroutines exported are clauses to the try subroutine.
The BLOCK will be evaluated and, if no error is throw, try will
return the result of the block.
"CLAUSES" are the subroutines below, which describe what to do in
the event of an error being thrown within BLOCK.
catch CLASS with BLOCK
This clauses will cause all errors that satisfy "$err->isa(CLASS)"
to be caught and handled by evaluating "BLOCK".
"BLOCK" will be passed two arguments. The first will be the error
being thrown. The second is a reference to a scalar variable. If
this variable is set by the catch block then, on return from the
catch block, try will continue processing as if the catch block was
never found. The error will also be available in $@.
To propagate the error the catch block may call "$err->throw"
If the scalar reference by the second argument is not set, and the
error is not thrown. Then the current try block will return with
the result from the catch block.
except BLOCK
When "try" is looking for a handler, if an except clause is found
"BLOCK" is evaluated. The return value from this block should be a
HASHREF or a list of key-value pairs, where the keys are class
names and the values are CODE references for the handler of errors
of that type.
otherwise BLOCK
Catch any error by executing the code in "BLOCK"
When evaluated "BLOCK" will be passed one argument, which will be
the error being processed. The error will also be available in $@.
Only one otherwise block may be specified per try block
finally BLOCK
Execute the code in "BLOCK" either after the code in the try block
has successfully completed, or if the try block throws an error
then "BLOCK" will be executed after the handler has completed.
If the handler throws an error then the error will be caught, the
finally block will be executed and the error will be re-thrown.
Only one finally block may be specified per try block
COMPATIBILITY
Moose exports a keyword called "with" which clashes with Error's. This
example returns a prototype mismatch error:
package MyTest;
use warnings;
use Moose;
use Error qw(:try);
(Thanks to "maik.hentsche@amd.com" for the report.).
CLASS INTERFACE
CONSTRUCTORS
The "Error" object is implemented as a HASH. This HASH is initialized
with the arguments that are passed to it's constructor. The elements
that are used by, or are retrievable by the "Error" class are listed
below, other classes may add to these.
-file
-line
-text
-value
-object
If "-file" or "-line" are not specified in the constructor arguments
then these will be initialized with the file name and line number where
the constructor was called from.
If the error is associated with an object then the object should be
passed as the "-object" argument. This will allow the "Error" package
to associate the error with the object.
The "Error" package remembers the last error created, and also the last
error associated with a package. This could either be the last error
created by a sub in that package, or the last error which passed an
object blessed into that package as the "-object" argument.
Error->new()
See the Error::Simple documentation.
throw ( [ ARGS ] )
Create a new "Error" object and throw an error, which will be
caught by a surrounding "try" block, if there is one. Otherwise it
will cause the program to exit.
"throw" may also be called on an existing error to re-throw it.
with ( [ ARGS ] )
Create a new "Error" object and returns it. This is defined for
syntactic sugar, eg
die with Some::Error ( ... );
record ( [ ARGS ] )
Create a new "Error" object and returns it. This is defined for
syntactic sugar, eg
record Some::Error ( ... )
and return;
STATIC METHODS
prior ( [ PACKAGE ] )
Return the last error created, or the last error associated with
"PACKAGE"
flush ( [ PACKAGE ] )
Flush the last error created, or the last error associated with
"PACKAGE".It is necessary to clear the error stack before exiting
the package or uncaught errors generated using "record" will be
reported.
$Error->flush;
OBJECT METHODS
stacktrace
If the variable $Error::Debug was non-zero when the error was
created, then "stacktrace" returns a string created by calling
"Carp::longmess". If the variable was zero the "stacktrace" returns
the text of the error appended with the filename and line number of
where the error was created, providing the text does not end with a
newline.
object
The object this error was associated with
file
The file where the constructor of this error was called from
line
The line where the constructor of this error was called from
text
The text of the error
$err->associate($obj)
Associates an error with an object to allow error propagation. I.e:
$ber->encode(...) or
return Error->prior($ber)->associate($ldap);
OVERLOAD METHODS
stringify
A method that converts the object into a string. This method may
simply return the same as the "text" method, or it may append more
information. For example the file name and line number.
By default this method returns the "-text" argument that was passed
to the constructor, or the string "Died" if none was given.
value
A method that will return a value that can be associated with the
error. For example if an error was created due to the failure of a
system call, then this may return the numeric value of $! at the
time.
By default this method returns the "-value" argument that was
passed to the constructor.
PRE-DEFINED ERROR CLASSES
Error::Simple This class can be used to hold simple error strings and values. It's constructor takes two arguments. The first is a text value, the second is a numeric value. These values are what will be returned by the overload methods. If the text value ends with "at file line 1" as $@ strings do, then this information will be used to set the "-file" and "-line" arguments of the error object. This class is used internally if an eval'd block die's with an error that is a plain string. (Unless $Error::ObjectifyCallback is modified)
$Error::ObjectifyCallback
This variable holds a reference to a subroutine that converts errors
that are plain strings to objects. It is used by Error.pm to convert
textual errors to objects, and can be overridden by the user.
It accepts a single argument which is a hash reference to named
parameters. Currently the only named parameter passed is 'text' which
is the text of the error, but others may be available in the future.
For example the following code will cause Error.pm to throw objects of
the class MyError::Bar by default:
sub throw_MyError_Bar
{
my $args = shift;
my $err = MyError::Bar->new();
$err->{'MyBarText'} = $args->{'text'};
return $err;
}
{
local $Error::ObjectifyCallback = \&throw_MyError_Bar;
# Error handling here.
}
MESSAGE HANDLERS
"Error" also provides handlers to extend the output of the "warn()"
perl function, and to handle the printing of a thrown "Error" that is
not caught or otherwise handled. These are not installed by default,
but are requested using the ":warndie" tag in the "use" line.
use Error qw( :warndie );
These new error handlers are installed in $SIG{__WARN__} and
$SIG{__DIE__}. If these handlers are already defined when the tag is
imported, the old values are stored, and used during the new code.
Thus, to arrange for custom handling of warnings and errors, you will
need to perform something like the following:
BEGIN {
$SIG{__WARN__} = sub {
print STDERR "My special warning handler: $_[0]"
};
}
use Error qw( :warndie );
Note that setting $SIG{__WARN__} after the ":warndie" tag has been
imported will overwrite the handler that "Error" provides. If this
cannot be avoided, then the tag can be explicitly "import"ed later
use Error;
$SIG{__WARN__} = ...;
import Error qw( :warndie );
EXAMPLE
The "__DIE__" handler turns messages such as
Can't call method "foo" on an undefined value at examples/warndie.pl line 16.
into
Unhandled perl error caught at toplevel:
Can't call method "foo" on an undefined value
Thrown from: examples/warndie.pl:16
Full stack trace:
main::inner('undef') called at examples/warndie.pl line 20
main::outer('undef') called at examples/warndie.pl line 23
SEE ALSO
See Exception::Class for a different module providing Object-Oriented
exception handling, along with a convenient syntax for declaring
hierarchies for them. It doesn't provide Error's syntactic sugar of
"try { ... }", "catch { ... }", etc. which may be a good thing or a bad
thing based on what you want. (Because Error's syntactic sugar tends to
break.)
Error::Exception aims to combine Error and Exception::Class "with
correct stringification".
TryCatch and Try::Tiny are similar in concept to Error.pm only
providing a syntax that hopefully breaks less.
KNOWN BUGS
None, but that does not mean there are not any.
AUTHORS
Graham Barr <gbarr@pobox.com> The code that inspired me to write this was originally written by Peter Seibel <peter@weblogic.com> and adapted by Jesse Glick <jglick@sig.bsh.com>. ":warndie" handlers added by Paul Evans <leonerd@leonerd.org.uk>
MAINTAINER
Shlomi Fish, <http://www.shlomifish.org/> .
PAST MAINTAINERS
Arun Kumar U <u_arunkumar@yahoo.com>
COPYRIGHT
Copyright (c) 1997-8 Graham Barr. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
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