NAME¶
Exception::Class::TryCatch - Syntactic try/catch sugar for use with
Exception::Class
VERSION¶
version 1.13
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 stringified 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;
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
SUPPORT¶
Bugs / Feature Requests¶
Please report any bugs or feature requests through the issue tracker at
<
https://github.com/dagolden/Exception-Class-TryCatch/issues>. You will
be notified automatically of any progress on your issue.
Source Code¶
This is open source software. The code repository is available for public review
and contribution under the terms of the license.
<
https://github.com/dagolden/Exception-Class-TryCatch>
git clone https://github.com/dagolden/Exception-Class-TryCatch.git
AUTHOR¶
David Golden <dagolden@cpan.org>
COPYRIGHT AND LICENSE¶
This software is Copyright (c) 2014 by David Golden.
This is free software, licensed under:
The Apache License, Version 2.0, January 2004