TryCatch(3pm)

1TryCatch(3)           User Contributed Perl Documentation          TryCatch(3)
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NAME

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

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

17        use TryCatch;
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19        sub foo {
20          my ($self) = @_;
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22          try {
23            die Some::Class->new(code => 404 ) if $self->not_found;
24            return "return value from foo";
25          }
26          catch (Some::Class $e where { $_->code > 100 } ) {
27          }
28        }
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SYNTAX

31       This module aims to give first class exception handling to perl via
32       'try' and 'catch' keywords. The basic syntax this module provides is
33       "try { # block }" followed by zero or more catch blocks. Each catch
34       block has an optional type constraint on it the resembles Perl6's
35       method signatures.
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37       Also worth noting is that the error variable ($@) is localised to the
38       try/catch blocks and will not leak outside the scope, or stomp on a
39       previous value of $@.
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41       The simplest case of a catch block is just
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43        catch { ... }
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45       where upon the error is available in the standard $@ variable and no
46       type checking is performed. The exception can instead be accessed via a
47       named lexical variable by providing a simple signature to the catch
48       block as follows:
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50        catch ($err) { ... }
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52       Type checking of the exception can be performed by specifing a type
53       constraint or where clauses in the signature as follows:
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55        catch (TypeFoo $e) { ... }
56        catch (Dict[code => Int, message => Str] $err) { ... }
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58       As shown in the above example, complex Moose types can be used,
59       including MooseX::Types style of type constraints
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61       In addition to type checking via Moose type constraints, you can also
62       use where clauses to only match a certain sub-condition on an error.
63       For example, assuming that "HTTPError" is a suitably defined TC:
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65        catch (HTTPError $e where { $_->code >= 400 && $_->code <= 499 } ) {
66          return "4XX error";
67        }
68        catch (HTTPError $e) {
69          return "other http code";
70        }
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72       would return "4XX error" in the case of a 404 error, and "other http
73       code" in the case of a 302.
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75       In the case where multiple catch blocks are present, the first one that
76       matches the type constraints (if any) will executed.
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BENEFITS

79       return. You can put a return in a try block, and it would do the right
80       thing - namely return a value from the subroutine you are in, instead
81       of just from the eval block.
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83       Type Checking. This is nothing you couldn't do manually yourself, it
84       does it for you using Moose type constraints.
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TODO

87       ·   Decide on "finally" semantics w.r.t return values.
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89       ·   Write some more documentation
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91       ·   Split out the dependancy on Moose
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AUTHOR

97       Ash Berlin <ash@cpan.org>
98

THANKS

100       Thanks to Matt S Trout and Florian Ragwitz for work on Devel::Declare
101       and various B::Hooks modules
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103       Vincent Pit for Scope::Upper that makes the return from block possible.
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105       Zefram for providing support and XS guidance.
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107       Xavier Bergade for the impetus to finally fix this module in 5.12.
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LICENSE

110       Licensed under the same terms as Perl itself.
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114perl v5.30.1                      2020-01-30                       TryCatch(3)