Chase Whitener

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.

Syntax::Keyword::Try - Catch exceptions in a familiar try and catch way. If you look no further, make use of this module or Try::Tiny! This module requires Perl v5.14 or better as it uses pluggable keywords.

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.

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 Syntax::Keyword::Try.

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

    #!/usr/bin/env perl
    use strict;
    use warnings;
    use Syntax::Keyword::Try;
    # since 5.14 is required, let's use those features
    use feature ':5.14';

    # 1
    try { say "0 plus 1 = ", increment(0); }
    catch { say "0 plus 1 = error"; }

    # error
    try { say "zero plus 1 = ", increment('zero'); }
    catch { say "zero plus 1 = error"; }

    # 1
    try { say "0 plus 1 = ", increment(0); }
    catch { say "0 plus 1 = error"; }

    sub increment {
        my $int = shift;
        die "That's not an int!" unless defined $int && $int =~ /^[0-9]+\z/;
        return $int+1;
    }

If you can't use Syntax::Keyword::Try, you can use the pure-Perl Try::Tiny instead:

    #!/usr/bin/env perl
    use strict;
    use warnings;
    use Try::Tiny qw(try catch);

    # 1
    try { print "0 plus 1 = ", increment(0), "\n"; }
    catch { print "0 plus 1 = error\n"; };

    # error
    try { print "zero plus 1 = ", increment('zero'), "\n"; }
    catch { print "zero plus 1 = error\n"; };

    # 1
    try { print "0 plus 1 = ", increment(0), "\n"; }
    catch { print "0 plus 1 = error\n"; };

    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.