Provided by: libexception-class-trycatch-perl_1.12-1_all 
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.