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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 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

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 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 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

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

AUTHORS
       Graham Barr 

       The code that inspired me to write this was originally written by Peter Seibel
        and adapted by Jesse Glick .

       ":warndie" handlers added by Paul Evans 

MAINTAINER
       Shlomi Fish 

PAST MAINTAINERS
       Arun Kumar U 

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 v5.20.2                                2015-10-23                                 Error(3pm)

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