| Exception::Class::Sugar - Syntactic sugar for use with Exception::Class |
Exception::Class::Sugar - Syntactic sugar for use with Exception::Class
use Exception::Class::Sugar;
# simple usage of catch()
eval { Exception::Class::Base->throw('error') };
catch my $err and warn $err->error;
# caught() is a synonym for catch()
eval { Exception::Class::Base->throw('error') };
if ( caught my $err ) {
if ($err->isa('this') { warn "this: $err->error" }
elsif ($err->isa('that') { warn "that: $err->error" }
else { $err->rethrow }
}
# use "try eval" to push exceptions onto a stack to catch later
try eval {
Exception::Class::Base->throw('error')
} and do {
# cleanup that might use "try/catch" again
};
catch my $err;
=head1 DESCRIPTION
Exception::Class::Sugar provides syntactic sugar for use with
the Exception::Class manpage using the familiar keywords try and catch. Its
primary objective is to allow users to avoid dealing directly with $@ by
ensuring that any exceptions caught in an eval are captured as
the Exception::Class manpage objects, whether they were thrown objects to begin with or
whether the error resulted from die. This means that users may immediately
use isa and various the Exception::Class manpage methods to process the exception.
In addition, this module provides for a method to push errors onto a hidden
error stack immediately after an eval so that cleanup code or other error
handling may also call eval without the original error in $@ being lost.
Inspiration for this module is due in part to Dave Rolsky's article "Exception Handling in Perl With Exception::Class" in The Perl Journal (Rolsky 2004).
The try/catch syntax used in this module does not use code reference
prototypes the way the Error.pm module does, but simply provides some
helpful functionality when used in combination with eval. As a result, it
avoids the complexity and dangers involving nested closures and memory leaks
inherent in Error.pm (Perrin 2003).
Rolsky (2004) notes that these memory leaks may not occur in recent versions of
Perl, but the approach used in Exception::Class::Sugar should be safe for all
versions of Perl as it leaves all code execution to the eval in the current
scope, avoiding closures altogether.
catch, caught
my $err = catch;
catch my $err;
caught my $err;
Returns an Exception::Class::Base object (or an object which is a
subclass of it) if an exception has been caught by eval or else
returns undef if no error exists. The exception is either popped
from a hidden error stack (see try) or, if the stack is empty, taken from
the current value of $@.
If the exception is not an Exception::Class::Base object (or subclass
object), an Exception::Class::Base object will be created using the string
contents of the exception. This means that calls to die will be wrapped and
may be treated as exception objects. Other objects caught will be stringfied
and wrapped likewise. Such wrapping will likely result in confusing stack
traces and the like, so any methods other than error used on
Exception::Class::Base objects caught should be used with caution.
catch is prototyped to take an optional scalar argument. When passed a
scalar variable, catch will also set that variable to the same value
returned. This allows for the catch my $err idiom without parentheses.
caught is a synonym for catch for syntactic convenience.
try
try eval {
#dangerous code
};
catch my $err;
Pushes the current error (C<$@>) onto a hidden error stack for later use by
C<catch>. C<try> uses a prototype that expects a single scalar so that it can
be used with eval without parentheses. As C<eval { BLOCK }> is an argument
to try, it will be evaluated just prior to C<try>, ensuring that C<try>
captures the correct error status. C<try> does not itself handle any errors --
it merely records the results of C<eval>. C<try { BLOCK }> will be interpreted
as passing a hash reference and will (probably) not compile.
try ignores the actual value returned by eval and always returns 1 -- as
trying is always successful regardless of whether the eval is successful
or not. This allows compound idioms like the following:
try eval {
# code
} and do {
# cleanup
};
catch my $err;
C<try> must always be properly bracketed with a matching C<catch> or unexpected
behavior may result when C<catch> pops the error off of the stack. C<try>
executes right after its C<eval>, so inconsistent usage of C<try> like the
following will work as expected:
try eval {
eval { die "inner" };
catch my $inner_err
die "outer" if $inner_err;
};
catch my $outer_err;
# handle $outer_err;
However, the following code is a problem:
# BAD EXAMPLE
try eval {
try eval { die "inner" };
die $@ if $@;
};
catch my $outer_err;
# handle $outer_err;
This code will appear to run correctly, but C<catch> gets the exception
from the inner C<try>, not the outer one, and there will still be an exception
on the error stack which will be caught by the next C<catch> in the program,
causing unexpected (and likely hard to track) behavior.
In short, if you use try, you must catch. The problem code above should
be rewritten as:
try eval {
try eval { die "inner" };
catch my $inner_err;
$inner_err->rethrow if $inner_err;
};
catch my $outer_err;
# handle $outer_err;
perrin. (2003), "Re: Re2: Learning how to use the Error module by example", (perlmonks.org), Available: http://www.perlmonks.org/index.pl?node_id=278900 (Accessed September 8, 2004).
Rolsky, D. (2004), "Exception Handling in Perl with Exception::Class", The Perl Journal, vol. 8, no. 7, pp. 9-13
- the Exception::Class manpage
- Error [but see (Perrin 2003) before using]
To install this module, type the following:
perl Build.PL ./Build ./Build test ./Build install
Though this is a simple module, this version is an alpha release and may contain bugs or have unexpected behaviors.
Please report bugs using the CPAN Request Tracker at http://rt.cpan.org/NoAuth/Bugs.html?Dist=Exception-Class-Sugar
David A. Golden (DAGOLDEN), dagolden@dagolden.com
Copyright (c) 2004 by David A. Golden
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
The full text of the license can be found in the LICENSE file included with this module.
| Exception::Class::Sugar - Syntactic sugar for use with Exception::Class |