Code Fast, die() Early, Throw Structured Exceptions

Live Fast,
Die Young,
Have A Good Looking Corpse

Code Fast,
die() Early,
Throw Structured Exceptions

Throw Structured Exceptions
John SJ Anderson
@genehack
03 Jan 2012

“Classic” Perl exception throwing

• Throw an exception with die()


• Or Carp::croak(), Carp::confess(), etc.



• TIMTOWTDI!



• TIMTOWTDI!

• Catch an exception with eval {}



• TIMTOWTDI!

• Catch an exception with eval {}

• Handle an exception by looking at $@

1 #! /usr/bin/perl
2
3 use strict;
4 use warnings;
5
6 eval { my $result = this_might_fail() };
7
8 if( $@ ) {
9 # handle the error here
10 }
11
12 sub this_might_fail {
13 die "FAILED!"
14 if rand() < 0.5;
15 }

Problems with “classic” Perl exceptions


• $@ can get clobbered



• $@ can get clobbered by code you don’t own




• $@ might be a false value





• If $@ is a string, you’re depending on duplicated
information, which will break.

Use Try::Tiny for
“semi-modern” Perl exceptions

Use Try::Tiny for
• Provides try{}/catch{}/finally{} blocks

Use Try::Tiny for
• Handles details of properly dealing with complexities
around $@

Use Try::Tiny for
around $@
• Lightweight and generally Just Works(tm).

Use Try::Tiny for
around $@
• Lightweight and generally Just Works(tm).
• N.b.: you have to end try{}/catch{} with a
semicolon. Don’t forget this!

Use Try::Tiny for
1 #! /usr/bin/perl
2
3 use strict;
4 use warnings;
5
6 use Try::Tiny;
7
8 try {
9 my $result = this_might_fail();
10 }
11 catch {
12 # handle the error here
13 };
14
15 sub this_might_fail {
16 die "FAILED!"
17 if rand() < 0.5;
18 }

Problems with

Wait, where’s the duplicated information?
1 my $answer;
2 try {
3 # any of these might throw an exception
4 my $this = this_might_fail();
5 my $that = something_else_might_fail();
6 $answer = combine_them( $this , $that );
7 }
8 catch {
9 # our error is in $_
10 if( $_ =~ /some error/ ) {
11 # handle some error
12 }
13 elsif( $_ =~ /another error/ ) {
14 # handle another error
15 }
16 else { # not sure what the problem is, just give up
17 confess( $_ );
18 }
19 };


• As soon as somebody “ﬁxes” the string in some die()
somewhere, you’ve potentially broken exception
handling


• As soon as somebody “ﬁxes” the string in some die()
somewhere, you’ve potentially broken exception
handling
• And you can’t even easily tell where, because it’s
probably in a regexp that doesn’t look at all like the
changed string


• Even if you have tests for the code in question, do you
really have test coverage on all your exception cases?


• Even if you have tests for the code in question, do you
really have test coverage on all your exception cases?
• (Almost certainly not. If you do, come write tests for
$WORK_PROJECT, we need the help...)

So what’s the solution?

• die() can also take a reference as an argument



• So you can die() with an object!




• Which means you can cram all sorts of useful information into your
exceptions




• Which means you can cram all sorts of useful information into your
exceptions

• And more importantly, handle them in a structured fashion that’s much less
brittle than string comparisons

A framework for structured exceptions:
Exception::Class
use Exception::Class (
    'MyException',

    'AnotherException' => { isa => 'MyException' },

    'YetAnotherException' => {
        isa         => 'AnotherException',
        description => 'These exceptions are related to IPC'
    },

    'ExceptionWithFields' => {
        isa    => 'YetAnotherException',
        fields => [ 'grandiosity', 'quixotic' ],
    },
);

A framework for structured exceptions:
Exception::Class
# try
eval { MyException->throw( error => 'I feel funny.' ) };

my $e;
# catch
if ( $e = Exception::Class->caught('MyException') ) {
    warn $e->error, "n", $e->trace->as_string, "n";
    warn join ' ', $e->euid, $e->egid, $e->uid, $e->gid, $e->pid;
    exit;
}
elsif ( $e = Exception::Class->caught('ExceptionWithFields') ) {
    $e->quixotic ? do_something_wacky() : do_something_sane();
}
else {
    $e = Exception::Class->caught();
    ref $e ? $e->rethrow : die $e;
}

Exception::Class Pros

• Nice declarative syntax


• Possible to declare detailed or simple exception class
hierarchies very simply


• Possible to declare detailed or simple exception class
hierarchies very simply
• Supports macro deﬁnitions to make throwing particular
exception types easier

Exception::Class Cons

• Not really designed for use with Try::Tiny

Exception::Class Cons

• Not really designed for use with Try::Tiny

• Based on Class::Data::Inheritable, not Moose

A Moose role for structured exceptions:
Throwable
package Redirect;
use Moose;
with 'Throwable';

has url => (is => 'ro');

...then later...

Redirect->throw({ url => $url });

Throwable Pros
• Implemented as a Moose role, so your exception
classes are just normal Moose classes that consume
the role

Throwable Pros
the role

• So you get the usual Moose-y good stuff around
attributes and methods and such.

Throwable Pros
the role

• So you get the usual Moose-y good stuff around
attributes and methods and such.
• Comes with a grab-bag of typical exception behaviors
(in Throwable::X), like stack traces, printf-ish
messages, etc.

Throwable Cons

?
So far, I haven’t really found any.

Throwable Cons

?
So far, I haven’t really found any.
(Of course, that doesn’t mean there aren’t any...)

Error Handling
Patterns & Anti-Patterns

Error Handling
• DO use exceptions instead of error ﬂags

Error Handling
• DO throw exceptions as early as possible

Error Handling
• DON’T catch exceptions unless you’re going to handle
them – just let them propagate upwards

Error Handling
• DON’T catch exceptions unless you’re going to handle
them – just let them propagate upwards
• DO design your web application-level error actions to
handle your business logic-level exceptions

Use exceptions instead of error ﬂags
1 sub some_catalyst_action :Local {
2 my( $self , $c ) = @_;
3 my $session = get_huge_session_object( $c->session );
4
5 my $validated_params = validate_request_params( $c->request->params )
6 or $c->detach( 'error' );
7
8 my $step_one_result = $c->model('BusinessLogic')->do_step_one( $session , $validated_params );
9 $c->detach( 'error' ) if $session->has_error();
10
11 my $step_two_result = $c->model('BusinessLogic')->do_step_two( $step_one_result, $session );
12 $c->detach( 'error' ) if $session->has_error();
13
14 $c->stash({
15 one => $step_one_result ,
16 two => $step_two_result ,
17 });
18 }


Please please please don’t write code like this!

• Forget just one of those checks and you’ve got a hard to track down
bug

bug

• Many times, $session was passed just to provide access to that
error ﬂag. Far too much information was being passed around for
no reason

bug

• Many times, $session was passed just to provide access to that
error ﬂag. Far too much information was being passed around for
no reason

• The error action gets no real info about what the problem was, or
it tries to pull it from $session itself (which has its own problems)


1 sub some_catalyst_action :Local {
2 my( $self , $c ) = @_;
3
4 try {
5 my $validated_params = validate_request_params( $c->request->params )
6
7 my $step_one_result = $c->model('BusinessLogic')->do_step_one( $session , $validated_params );
8
9 my $step_two_result = $c->model('BusinessLogic')->do_step_two( $step_one_result, $session );
10 }
11 catch { $c->detach( 'error' , [ $_ ] ) };
12
13 $c->stash({
14 one => $step_one_result ,
15 two => $step_two_result ,
16 });
17 }

Throw exceptions as early as
possible

possible
• If you’re going to throw an exception because you
didn’t get passed something, do it ASAP.

possible
• If you’re going to throw an exception because you
didn’t get passed something, do it ASAP.

• In general, the quicker you can die(), the better –
because it reduces the amount of code that might
contain the bug.

Don’t catch exceptions
except to handle them

• Most of the time, your business logic code is going to
throw exceptions, not catch them

• If you do catch an exception, you should be trying to ﬁx
the problem.

• If you do catch an exception, you should be trying to ﬁx
the problem.
• Don’t catch exceptions just to munge them or log them
and re-throw them. Munge them or log them before
you throw them.

Web application error actions
should handle exceptions
1 sub error :Private {
2 my( $self , $c , $error ) = @_;
3
4 my $message = 'An unexpected error happened.';
5
6 # NOTE: duck typing
7 $message = $error->user_visible_message
8 if( $error->has_user_visible_message and ! $error->is_private );
9
10 $c->stash({
11 message => $message ,
12 template => 'error',
13 });
14 }

Web application error actions
should handle exceptions
1 sub error :Private {
2 my( $self , $c , $error ) = @_;
3
4 my $message = 'An unexpected error happened.';
5
6 # NOTE: duck typing
7 $message = $error->user_visible_message
8 if( $error->has_user_visible_message and ! $error->is_private );
9
10 $c->stash({
11 message => $message ,
12 template => 'error',
13 });
14 }

(again, not the best example ever...)

Further reading
• Throwable::X: common behavior for thrown exceptions
(<http://rjbs.manxome.org/rubric/entry/1860>)

• Exceptionally Extensible Exceptions
(<http://advent.rjbs.manxome.org/2010/2010-12-03.html>)

• Structured Data and Knowing versus Guessing

(<http://www.modernperlbooks.com/mt/2010/10/structured-data-and-knowing-versus-guessing.html>)

Thanks for your time
this evening!

Code Fast, die() Early, Throw Structured Exceptions

More Related Content

Viewers also liked (16)

Similar to Code Fast, die() Early, Throw Structured Exceptions (20)

More from John Anderson (20)

Recently uploaded (20)

Code Fast, die() Early, Throw Structured Exceptions

Editor's Notes