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Man Pages


Manual Reference Pages  -  TRYCATCH (3)

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NAME

TryCatch - first class try catch semantics for Perl, without source filters.

CONTENTS

DESCRIPTION

This module aims to provide a nicer syntax and method to catch errors in Perl, similar to what is found in other languages (such as Java, Python or C++). The standard method of using eval {}; if ($@) {} is often prone to subtle bugs, primarily that its far too easy to stomp on the error in error handlers. And also eval/if isn’t the nicest idiom.

SYNOPSIS



 use TryCatch;

 sub foo {
   my ($self) = @_;

   try {
     die Some::Class->new(code => 404 ) if $self->not_found;
     return "return value from foo";
   }
   catch (Some::Class $e where { $_->code > 100 } ) {
   }
 }



SYNTAX

This module aims to give first class exception handling to perl via ’try’ and ’catch’ keywords. The basic syntax this module provides is try { # block } followed by zero or more catch blocks. Each catch block has an optional type constraint on it the resembles Perl6’s method signatures.

Also worth noting is that the error variable ($@) is localised to the try/catch blocks and will not leak outside the scope, or stomp on a previous value of $@.

The simplest case of a catch block is just



 catch { ... }



where upon the error is available in the standard $@ variable and no type checking is performed. The exception can instead be accessed via a named lexical variable by providing a simple signature to the catch block as follows:



 catch ($err) { ... }



Type checking of the exception can be performed by specifing a type constraint or where clauses in the signature as follows:



 catch (TypeFoo $e) { ... }
 catch (Dict[code => Int, message => Str] $err) { ... }



As shown in the above example, complex Moose types can be used, including MooseX::Types style of type constraints

In addition to type checking via Moose type constraints, you can also use where clauses to only match a certain sub-condition on an error. For example, assuming that HTTPError is a suitably defined TC:



 catch (HTTPError $e where { $_->code >= 400 && $_->code <= 499 } ) {
   return "4XX error";
 }
 catch (HTTPError $e) {
   return "other http code";
 }



would return 4XX error in the case of a 404 error, and other http code in the case of a 302.

In the case where multiple catch blocks are present, the first one that matches the type constraints (if any) will executed.

BENEFITS

<B>returnB>. You can put a return in a try block, and it would do the right thing - namely return a value from the subroutine you are in, instead of just from the eval block.

<B>Type CheckingB>. This is nothing you couldn’t do manually yourself, it does it for you using Moose type constraints.

TODO

o Decide on finally semantics w.r.t return values.
o Write some more documentation
o Split out the dependancy on Moose

SEE ALSO

MooseX::Types, Moose::Util::TypeConstraints, Parse::Method::Signatures.

AUTHOR

Ash Berlin <ash@cpan.org>

THANKS

Thanks to Matt S Trout and Florian Ragwitz for work on Devel::Declare and various B::Hooks modules

Vincent Pit for Scope::Upper that makes the return from block possible.

Zefram for providing support and XS guidance.

Xavier Bergade for the impetus to finally fix this module in 5.12.

LICENSE

Licensed under the same terms as Perl itself.
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perl v5.20.3 TRYCATCH (3) 2013-03-22

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