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Error(3pm)            User Contributed Perl Documentation           Error(3pm)



NAME
       Error - Error/exception handling in an OO-ish way

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 subrou-
           tines 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

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 cre-
           ated, 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 over-
       load methods.

       If the text value ends with "at file line 1" as $@ strings do, then
       this infomation 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 overrided by the user.

       It accepts a single argument which is a hash reference to named parame-
       ters.  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 can-
       not 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

KNOWN BUGS
       None, but that does not mean there are not any.

AUTHORS
       Graham Barr <gbarrATpobox.com>

       The code that inspired me to write this was originally written by Peter
       Seibel <peterATweblogic.com> and adapted by Jesse Glick
       <jglickATsig.com>.

       ":warndie" handlers added by Paul Evans <leonerdATleonerd.uk>

MAINTAINER
       Shlomi Fish <shlomifATiglu.il>

PAST MAINTAINERS
       Arun Kumar U <u_arunkumarATyahoo.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.



perl v5.8.8                       2007-12-02                        Error(3pm)