NAME

    Exceptions - Documentation for exception handling in Perl.

DESCRIPTION

    This module doesn't do anything, it exists solely to document how to
    handle exceptions in Perl.

WHY?

    This module was originally released in 1996, but it hasn't been
    installable or usable in any fashion since then. Many other
    alternatives have cropped up over the years to make exception handling
    much easier. If you want to skip the explanations below, then you
    should look directly at some of the modules that make exception
    handling dead simple.

    Try::Tiny - Catch exceptions in a familiar try and catch way. If you
    look no further, make use of this module!

    With a good way to catch exceptions, now you need exception types so
    you can re-throw exceptions when they're something that should be
    handled elsewhere.

      * Throwable and Throwable::SugarFactory

      * Exception::Class

      * Mojo::Exception

AN EXCEPTION

    An exception is what happens anytime your program's execution exits
    unexpectedly. Let's start with a simple example.

        #!/usr/bin/env perl
        use strict;
        use warnings;
    
        print "0 plus 1 = ", increment(0), "\n"; # 1
        print "zero plus 1 = ", increment('zero'), "\n"; # Exception
        print "0 plus 1 = ", increment(0), "\n"; # never executes
    
        sub increment {
            my $int = shift;
            die "That's not an int!" unless defined $int && $int =~ /^[0-9]+\z/;
            return $int+1;
        }

    This will result in the following output:

        $ perl increment.pl
        0 plus 1 = 1
        That's not an int! at foo.pl line 11.

    The first line prints 0 plus 1 = 1\n as expected. The second line,
    however, dies in a way that we can't recover from which prevents the
    rest of our program from doing any further execution. So, we must
    handle our exceptions!

A HANDLED EXCEPTION

    The only way you can handle an exception is to wrap the code that could
    die in an eval block. This sounds simple enough, but there are some
    gotchas that lead many developers to do this incorrectly.

    The correct way to handle an exception requires that you understand how
    to preserve the global $@ variable. Please see "BACKGROUND" in
    Try::Tiny for a great explanation of this problem.

    Let's look at our previous simple application with error handling using
    eval.

        #!/usr/bin/env perl
        use strict;
        use warnings;
    
        my $value;
        my $error = do { # catch block
            local $@;
            eval { $value = increment(0) }; # try
            $@;
        };
    
        print "0 plus 1 = ";
        if ($error) {
            print "error";
        }
        else {
            print $value;
        }
        print "\n"; # 1
    
        $value = undef;
        $error = undef;
        $error = do { # catch block
            local $@;
            eval { $value = increment('zero') }; # try
            $@;
        };
    
        print "zero plus 1 = ";
        if ($error) {
            print "error";
        }
        else {
            print $value;
        }
        print "\n"; # error
    
        $value = undef;
        $error = undef;
        $error = do { # catch block
            local $@;
            eval { $value = increment(0) }; # try
            $@;
        };
    
        print "0 plus 1 = ";
        if ($error) {
            print "error";
        }
        else {
            print $value;
        }
        print "\n"; # 1
    
        sub increment {
            my $int = shift;
            die "That's not an int!" unless defined $int && $int =~ /^[0-9]+\z/;
            return $int+1;
        }

    As you can see, it gets quite ugly and cumbersome to handle exceptions
    this way. Don't let that scare you away from Perl, though. Keep reading
    and be happy!

THE SOLUTION

    Lucky for us, Perl is an awesome language where the community provides
    many solutions to common tasks for us. One such solution is Try::Tiny.

    If you get nothing else out of this document, let it be that using
    Try::Tiny will save you time and heartache.

        #!/usr/bin/env perl
        use strict;
        use warnings;
        use Try::Tiny qw(try catch);
    
        print "0 plus 1 = ", try { increment(0) } catch { "error" };
        print "\n"; # 1
    
        print "zero plus 1 = ", try { increment('zero') } catch { "error" };
        print "\n"; # error
    
        print "0 plus 1 = ", try { increment(0) } catch { "error" };
        print "\n"; # 1
    
        sub increment {
            my $int = shift;
            die "That's not an int!" unless defined $int && $int =~ /^[0-9]+\z/;
            return $int+1;
        }

AUTHOR

    Chase Whitener <capoeirab@cpan.org>

LICENSE AND COPYRIGHT

    Copyright 1996 by Peter Seibel <pseibel@cpan.org>. This original
    release was made without an attached license.

    Copyright 2016 by Chase Whitener <capoeirab@cpan.org>. This re-release
    contains none of the original code or structure and is thus re-released
    under the same license as Perl itself.

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.