NAME
Exception::Class::TryCatch - Syntactic try/catch sugar for use with
Exception::Class
VERSION
This documentation describes version 1.12.
SYNOPSIS
use Exception::Class::TryCatch;
# simple usage of catch()
eval { Exception::Class::Base->throw('error') };
catch my $err and warn $err->error;
# catching only certain types or else rethrowing
eval { Exception::Class::Base::SubClass->throw('error') };
catch( my $err, ['Exception::Class::Base', 'Other::Exception'] )
and warn $err->error;
# catching and handling different types of errors
eval { Exception::Class::Base->throw('error') };
if ( catch my $err ) {
$err->isa('this') and do { handle_this($err) };
$err->isa('that') and do { handle_that($err) };
}
# use "try eval" to push exceptions onto a stack to catch later
try eval {
Exception::Class::Base->throw('error')
};
do {
# cleanup that might use "try/catch" again
};
catch my $err; # catches a matching "try"
DESCRIPTION
Exception::Class::TryCatch provides syntactic sugar for use with
Exception::Class 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
Exception::Class 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 Exception::Class 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::TryCatch
should be safe for all versions of Perl as it leaves all code execution
to the "eval" in the current scope, avoiding closures altogether.
USAGE
"catch"
# zero argument form
my $err = catch;
# one argument forms
catch my $err;
my $err = catch( [ 'Exception::Type', 'Exception::Other::Type' ] );
# two argument form
catch my $err, [ 'Exception::Type', 'Exception::Other::Type' ];
Returns an "Exception::Class::Base" object (or an object which is a
subclass of it) if an exception has been caught by "eval". If no
exception was thrown, it returns "undef" in scalar context and an empty
list in list context. 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 up to two optional scalar arguments. The
single argument form has two variations.
* If the argument is a reference to an array, any exception caught
that is not of the same type (or a subtype) of one of the classes
listed in the array will be rethrown.
* If the argument is not a reference to an array, "catch" will set the
argument to the same value that is returned. This allows for the
"catch my $err" idiom without parentheses.
In the two-argument form, the first argument is set to the same value as
is returned. The second argument must be an array reference and is
handled the same as as for the single argument version with an array
reference, as given above.
"caught" (DEPRECATED)
"caught" is a synonym for "catch" for syntactic convenience.
NOTE: Exception::Class version 1.21 added a "caught" method of its own.
It provides somewhat similar functionality to this subroutine, but with
very different semantics. As this class is intended to work closely with
Exception::Class, the existence of a subroutine and a method with the
same name is liable to cause confusion and this method is deprecated and
may be removed in future releases of Exception::Class::TryCatch.
This method is no longer exported by default.
"try"
# void context
try eval {
# dangerous code
};
do {
# cleanup code can use try/catch
};
catch my $err;
# scalar context
$rv = try eval { return $scalar };
# list context
@rv = try [ eval { return @array } ];
Pushes the current error ($@) onto a hidden error stack for later use by
"catch". "try" uses a prototype that expects a single scalar so that it
can be used with eval without parentheses. As "eval { BLOCK }" is an
argument to try, it will be evaluated just prior to "try", ensuring that
"try" captures the correct error status. "try" does not itself handle
any errors -- it merely records the results of "eval". "try { BLOCK }"
will be interpreted as passing a hash reference and will (probably) not
compile. (And if it does, it will result in very unexpected behavior.)
Since "try" requires a single argument, "eval" will normally be called
in scalar context. To use "eval" in list context with "try", put the
call to "eval" in an anonymous array:
@rv = try [ eval {return @array} ];
When "try" is called in list context, if the argument to "try" is an
array reference, "try" will dereference the array and return the
resulting list.
In scalar context, "try" passes through the scalar value returned by
"eval" without modifications -- even if that is an array reference.
$rv = try eval { return $scalar };
$rv = try eval { return [ qw( anonymous array ) ] };
Of course, if the eval throws an exception, "eval" and thus "try" will
return undef.
"try" must always be properly bracketed with a matching "catch" or
unexpected behavior may result when "catch" pops the error off of the
stack. "try" executes right after its "eval", so inconsistent usage of
"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 "catch" gets the exception
from the inner "try", not the outer one, and there will still be an
exception on the error stack which will be caught by the next "catch" in
the program, causing unexpected (and likely hard to track) behavior.
In short, if you use "try", you must have a matching "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;
BUGS
Please report any bugs or feature using the CPAN Request Tracker. Bugs
can be submitted through the web interface at
<http://rt.cpan.org/Dist/Display.html?Queue=Exception-Class-TryCatch>
When submitting a bug or request, please include a test-file or a patch
to an existing test-file that illustrates the bug or desired feature.
REFERENCES
1. 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).
2. Rolsky, D. (2004), "Exception Handling in Perl with
Exception::Class", *The Perl Journal*, vol. 8, no. 7, pp. 9-13
SEE ALSO
* Exception::Class
* Error -- but see (Perrin 2003) before using
AUTHOR
David A. Golden (DAGOLDEN)
COPYRIGHT AND LICENSE
Copyright (c) 2004-2008 by David A. Golden. All rights reserved.
Licensed under Apache License, Version 2.0 (the "License"). You may not
use this file except in compliance with the License. A copy of the
License was distributed with this file or you may obtain a copy of the
License from http://www.apache.org/licenses/LICENSE-2.0
Files produced as output though the use of this software, shall not be
considered Derivative Works, but shall be considered the original work
of the Licensor.
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
