| Error - Error/exception handling in an OO-ish way |
Error - Error/exception handling in an OO-ish way
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.
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 surprisedThe 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.
Error exports subroutines to perform exception handling. These will be exported if the :try tag is used in the use line.
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.
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.
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.
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
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
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.).
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
-objectIf -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.
See the Error::Simple documentation.
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.
Create a new Error object and returns it. This is defined for syntactic sugar, eg
die with Some::Error ( ... );Create a new Error object and returns it. This is defined for syntactic sugar, eg
record Some::Error ( ... )
and return;Return the last error created, or the last error associated with 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;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.
The object this error was associated with
The file where the constructor of this error was called from
The line where the constructor of this error was called from
The text of the error
Associates an error with an object to allow error propagation. I.e:
$ber->encode(...) or
return Error->prior($ber)->associate($ldap);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.
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.
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)
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.
}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 imported later
use Error;
$SIG{__WARN__} = ...;
import Error qw( :warndie );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 23See 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.
None, but that does not mean there are not any.
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>
Shlomi Fish, http://www.shlomifish.org/ .
Arun Kumar U <u_arunkumar@yahoo.com>
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.
| Error - Error/exception handling in an OO-ish way |