Sub::ForceEval - eval subroutines, re-throw exceptions if there is an eval; otherwise cluck and return undef.

    # you may just want your death recorded...
    #
    # if foo dies in an eval then $@ will be re-thrown, 
    # otherwise foo will cluck, return undef, and keep 
    # going.

    use Sub::ForceEval;

    sub foo :ForceEval
    {
       ...
    }

    # a bare call to foo() in the main code will cluck.

    foo();

    # the exception is re-thrown here, however, since
    # bletch was called from within an eval.

    eval { bletch() };

    sub bletch { bar() }

    sub bar { foo() }




    # ... or you may want an exceptional death.
    #
    # the default in MyClass is to have ForceEval 
    # call My::Class::Default->constructor( $@ )
    # before re-throwing, marine re-throws 
    # Dive::Dive->dive( $@ ).

    package MyClass;

    use Sub::ForceEval qw( My::Class::Default->constructor );

    sub marine :ForceEval( 'Dive::Dive->dive' );

    # then again, you may just want to record or
    # tidy up the message. in this case, you can pass
    # in a function without the '->' separator and 
    # it'll be callsed as function( $@ ).

    use Sub::ForceEval qw( Some::Package::function );

Subroutines that are marked with the ForceEval attribute check at runtime if there is an eval on the stack when the call dies. If an eval is found then the die is propagated to it via die $@; if not then cluck (see Carp) is called and the die is trapped.

The stack is only checked if something dies, so there is relatively little overhead in using the attribute: just an eval and intermediate storage of the return values.

Note that this inludes anything that dies for any reason even if the death is not intended as an OO 'exception'. This can be helpful for long-lived processes that need to ensure survival. It can also be handy for subs that call modules which use Fatal: all of the fatalities can be guaranteed to be gracefully handled.

If exception objects are preferred to flat $@ values then a constructor can be provided with the use. This will be broken into class and method portions and called to construct an object from the exception; individual subs can also provide a constructor.

Use the module and add the :ForceEval attribute to a subroutine:

    use Sub::ForceEval;

    sub foo :ForceEval { ...}

If foo dies within an eval then "die $@" is used to propagate the exception.

Each package that uses ForceEval can have its own setting. There is currently no way to set a global default.

Package default exception

Passing a constructor to "use" will wrap all $@ via the constructor for subroutines in that package:

    use Sub::ForceEval qw( Exceptional::Class->construct );

The literal '->' is text, it does not mean that the constructor needs to return a subref.

This will be broken into $class of Exceptional::Class and method of "construct", which are then called as:

    die $class->$method( $@ );

so that whatever they return will be passed as the exception.

Subroutine-specific exception

Passing a constructor to the ForceEval attribute will use that instead of any package default:

    sub frobnicate :ForceEval qw( Exceptional::File->handler )
    {
        ...
    }

leaves $class and $method for the subroutine set to "Exceptional::File" and "handler".

If all you want to do is log or munge the errors, then a simple subroutine may do just as well. These are used via:

  use Sub::ForceEval qw( My::function );

or

  sub foo :ForceEval qw( Some::Package::munge_error )
  {
    ...
  }

Functions are checked via $package->can( $name ), defaulting to a stub that passes back $@ as-is with a warning that "$package cannot $name".

using "sub"

Due to the handling by Attribute::Handlers, adding ForceEval to AUTOLOAD, DESTROY, BEGIN, CHECK, or INIT blocks requires that they have a 'sub' prefix in the code.

Working code:

  sub AUTOLOAD  :ForceEval
  {
    ...
  }

This will fail since it lacks the "sub":

  AUTOLOAD  :ForceEval
  {
    ...
  }

$AUTOLOAD

The autoload in Sub::ForceEval is installed into the Eval'ed sub's package. This means that all AUTOLOADS that use ForceEval in the program will share a single $AUTOLOAD. This is not normally an issue since only one AUTOLOAD at a time is called and daisy-chaining them depends on their having a common value anyway.

"Missing eval for '$name'", $@

An :ForceEval subroutine was called from a context where exceptions would not be caught by any surrounding eval. This uses Carp::cluck to complain about the fact and keeps going.

"Warning: 'Package <package>' cannot '<name>'

"Warning: 'Class <package>' cannot '<name>'

Breaking up the handler argument on '->' or the final '::' gives a package and name. These are checked via

  $package->can( $name )

at runtime, prior to dispatch. If the given package does not have the name in it (or one of its base classes in OO) then Sub::ForceEval logs the warning and returns $@ as-is.

Sub::ForceEval requires no configuration files or environment variables.

  strict

  Carp

  Symbol

  version

  Attribute::Handlers 

None reported.

No bugs have been reported.

Some parts of this are impossible to test since detecting that the module incorrectly detected an existing eval requires running it in an eval.

Please report any bugs or feature requests to bug-sub-ForceEval@rt.cpan.org, or through the web interface at http://rt.cpan.org.

Steven Lembark <lembark@wrkhors.com>

Damian Conway

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See perlartistic.

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