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

    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 \-text => "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

The CWError package provides two interfaces. Firstly CWError provides a procedural interface to exception handling. Secondly CWError is a base class for errors/exceptions that can either be thrown, for subsequent catch, or can simply be recorded.

Errors in the class CWError should not be thrown directly, but the user should throw errors from a sub-class of CWError.

CWError exports subroutines to perform exception handling. These will be exported if the CW:try tag is used in the CWuse line.

try BLOCK CLAUSES

CWtry 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. CWCLAUSES 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 CW$err->isa(CLASS) to be caught and handled by evaluating CWBLOCK. CWBLOCK will be passed two arguments. The first will be the error being thrown. The second is a reference to a scalar variable. If this variable is set by the catch block then, on return from the catch block, try will continue processing as if the catch block was never found. To propagate the error the catch block may call CW$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 CWtry is looking for a handler, if an except clause is found CWBLOCK 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 CWBLOCK When evaluated CWBLOCK will be passed one argument, which will be the error being processed. Only one otherwise block may be specified per try block

finally BLOCK

Execute the code in CWBLOCK either after the code in the try block has successfully completed, or if the try block throws an error then CWBLOCK 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

The CWError 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 CWError class are listed below, other classes may add to these.

        \-file
        \-line
        \-text
        \-value
        \-object

If CW-file or CW-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 CW-object argument. This will allow the CWError package to associate the error with the object.

The CWError 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 CW-object argument.

throw ( [ ARGS ] )

Create a new CWError object and throw an error, which will be caught by a surrounding CWtry block, if there is one. Otherwise it will cause the program to exit. CWthrow may also be called on an existing error to re-throw it.

with ( [ ARGS ] )

Create a new CWError object and returns it. This is defined for syntactic sugar, eg

    die with Some::Error ( ... );

record ( [ ARGS ] )

Create a new CWError object and returns it. This is defined for syntactic sugar, eg

    record Some::Error ( ... )
        and return;

prior ( [ PACKAGE ] )

Return the last error created, or the last error associated with CWPACKAGE

stacktrace

If the variable CW$Error::Debug was non-zero when the error was created, then CWstacktrace returns a string created by calling CWCarp::longmess. If the variable was zero the CWstacktrace 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

stringify

A method that converts the object into a string. This method may simply return the same as the CWtext method, or it may append more information. For example the file name and line number. By default this method returns the CW-text argument that was passed to the constructor, or the string CW"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 CW$! at the time. By default this method returns the CW-value argument that was passed to the constructor.

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 CWat file line 1 as $@ strings do, then this infomation will be used to set the CW-file and CW-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.

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>.

Arun Kumar U <u_arunkumar@yahoo.com>