Introduction: The current state of play

Perl has very min­i­mal” sup­port for object-​oriented (OO) pro­gram­ming out of the box by its own admis­sion. It’s class-​based but class­es are just pack­ages used dif­fer­ent­ly. Objects are just data struc­tures blessed into a class, meth­ods are just sub­rou­tines whose first argu­ment is an object or class name, and attributes/​properties are often just the key-​value pair of a hash stored in the object. (This last is a fea­ture shared with JavaScript, whose prototype-​based objects are just col­lec­tions of key-​value pairs with the keys addressed as prop­er­ties.) You’ve got poly­mor­phism, inher­i­tance, and it’s up to you to enforce encap­su­la­tion.

This can take a lot of work to use effec­tive­ly. To help address that, sev­er­al sys­tems have been devel­oped over the years to reduce boil­er­plate and pro­vide mod­ern (or post­mod­ern”) OO fea­tures that devel­op­ers from oth­er lan­guages expect. My favorite for a while has been Moo: it’s got the fea­tures I need 90% of the time like built-​in con­struc­tors, roles (an alter­na­tive to com­po­si­tion through inher­i­tance), attrib­ut­es, type val­i­da­tion, and method mod­i­fiers for enhanced poly­mor­phism. And if I need to dig around in the guts of class­es, attrib­ut­es, and the like I can always upgrade to Moo’s big broth­er Moose and its meta-​object pro­to­col with min­i­mal effort.

Corinna, Object::Pad, and porting dbcritic

But there’s a new kid on the block. Curtis Ovid” Poe has been spear­head­ing Corinna, an effort to bring effec­tive OO to the Perl core and leapfrog [empha­sis his] the capa­bil­i­ties of many OO lan­guages today.” No CPAN mod­ules, no chain of depen­den­cies; just sol­id OO fea­tures and syn­tax built-​in. And while Corinna is a ways off from ship­ping, Paul LeoNerd” Evans (maybe I should get a cool nick­name too?) has been imple­ment­ing some of these ideas as new Perl key­word syn­tax in his Object::Pad module.

Both Ovid and LeoNerd have been ask­ing devel­op­ers to try out Object::Pad, not just as a new toy, but to get feed­back on what works and what needs to be added. So I thought I’d try port­ing an old­er small Moo-​based project named dbcrit­ic to this new real­i­ty. In the process, I learned some of the advan­tages and dis­ad­van­tages of work­ing with Object::Pad. Hopefully, this can inform both it and Corinna’s evo­lu­tion as well as oth­er curi­ous devel­op­ers’ eval­u­a­tions. You can fol­low my cod­ing efforts in this GitHub branch.

First, the mar­quee result: the code for App::DBCritic (the class I start­ed with) is clean­er and short­er, with 33 lines shaved off so far. Mainly this is due to Object::Pad’s more con­cise attribute syn­tax (called slots” in its doc­u­men­ta­tion) and lack of explic­it sup­port for Moo’s attribute coer­cion. I only used the lat­ter for one attribute in the Moo ver­sion and I’m not sure it worked par­tic­u­lar­ly well, so it was­n’t hard to jet­ti­son. But if your code sup­ports coer­cions exten­sive­ly, you’ll have to look into Object::Pad’s BUILD or ADJUST phase blocks for now.

Before, a Moo attribute with var­i­ous options:

has schema => (
    is        => 'ro',
    coerce    => 1,
    lazy      => 1,
    default   => \&_build_schema,
    coerce    => \&_coerce_schema,
    predicate => 1,

After, an Object::Pad slot. No coer­cion and builder code is han­dled in a lat­er ADJUST block:

has $schema :reader :param = undef;

Speaking of ADJUST blocks, it took a lit­tle bit of insight from the #perl IRC chan­nel to real­ize that they were the appro­pri­ate place for set­ting slot defaults that are com­put­ed from oth­er slots. Previously I was using a maze of depen­den­cies mix­ing Moo lazy attrib­ut­es and builder meth­ods. Clarifying the main set of option­al con­struc­tor argu­ments into a sin­gle ADJUST block helped untan­gle things, so this might be an indi­ca­tion that lazy attrib­ut­es are an antipat­tern when try­ing to write clean code. It’s also worth not­ing that Object::Pad ADJUST blocks run on object con­struc­tion, where­as Moo lazy attrib­ut­es are only built when need­ed. This tends to mat­ter for data­base access.

The ADJUST block for the $schema slot:

    my @connect_info = ( $dsn, $username, $password );
    if ($class_name and eval "require $class_name") {
        $schema = $class_name->connect(@connect_info);
    elsif ( not ( blessed($schema) and $schema->isa('DBIx::Class::Schema') ) ) {
        local $SIG{__WARN__} = sub {
            if ( $_[0] !~ / has no primary key at /ms ) {
                print {*STDERR} $_[0];
        $schema = App::DBCritic::Loader->connect(@connect_info);
    croak 'No schema defined' if not $schema;

Object::Pad’s slots have one great advan­tage over Moo and Moose attrib­ut­es: they direct­ly sup­port Perl array and hash data struc­tures, while the lat­ter only sup­ports scalars and ref­er­ences con­tained in scalars. This means meth­ods in your class can elim­i­nate a deref­er­enc­ing step, again lead­ing to clean­er code. I used this specif­i­cal­ly in the @violations array and %elements hash slots and was very pleased with the results.

The @violations and %elements slots and their ADJUST blocks:

has %elements;

    %elements = (
        Schema       => [$schema],
        ResultSource => [ map { $schema->source($_) } $schema->sources ],
        ResultSet    => [ map { $schema->resultset($_) } $schema->sources ],

has @violations;

    @violations = map { $self->_policy_loop( $_, $elements{$_} ) }
        keys %elements;

method violations { wantarray ? @violations : \@violations }


I did have some devel­op­ment life­cy­cle issues with Object::Pad, but they’re main­ly a result of its future-​facing syn­tax. I had to give up using perltidy and perlcritic in my build and test phas­es, respec­tive­ly: perltidy does­n’t under­stand slot attrib­ut­es like :reader and :param and will emit an error file (but code still com­piles), and sev­er­al of the perlcritic poli­cies I use report prob­lems because its PPI pars­er does­n’t rec­og­nize the new syn­tax. I could add excep­tions in the perlcriticrc file and lit­ter my code with more ## no critic anno­ta­tions than it already had, but at this point, it was eas­i­er to just dis­able it entirely.

Another thing I had to dis­able for now was my Dist::Zilla::Plugin::Test::UnusedVars-gen­er­at­ed Test::Vars test for detect­ing unused vari­ables, as it reports mul­ti­ple fail­ures for the hid­den @(Object::Pad/slots) vari­able. It does have options for ignor­ing cer­tain vari­ables, though, so I can explore using those and pos­si­bly file a pull request to ignore that vari­able by default.

Conclusion: The future looks bright

Overall I’m sat­is­fied with Object::Pad and by exten­sion some of the syn­tax that Corinna will intro­duce. I’m going to try port­ing the rest of dbcrit­ic and see if I can work around the issues I list­ed above with­out giv­ing up the kwali­tee improve­ment tools I’m used to. I’ll post my find­ings if I feel it mer­its anoth­er blog.

What do you think? Is this the future of object-​oriented Perl? Let me know in the com­ments below.

iceberg in body of water

We have a huge code­base of over 700,000 lines of Perl spread across a cou­ple dozen Git repos­i­to­ries at work. Sometimes refac­tor­ing is easy if the class­es and meth­ods involved are con­fined to one of those repos, but last week we want­ed to rename a method that was poten­tial­ly used across many of them with­out hav­ing to QA and launch so many changes. After get­ting some help from Dan Book and Ryan Voots on the #perl IRC chan­nel, I arrived at the fol­low­ing solution.

First, if all you want to do is alias the new method call to the old while mak­ing the least amount of changes, you can just do this:

*new_method = \&old_method;

This takes advan­tage of Perl’s type­globs by assign­ing to the new method­’s name in the sym­bol table a ref­er­ence (indi­cat­ed by the \ char­ac­ter) to the old method. Methods are just sub­rou­tines in Perl, and although you don’t need the & char­ac­ter when call­ing one, you do need it if you’re pass­ing a sub­rou­tine as an argu­ment or cre­at­ing a ref­er­ence, as we’re doing above.

I want­ed to do a bit more, though. First, I want­ed to log the calls to the old method name so that I could track just how wide­ly it’s used and have a head start on renam­ing it else­where in our code­base. Also, I did­n’t want to fill our logs with those calls — we have enough noise in there already. And last­ly, I want­ed future calls to go direct­ly to the new method name with­out adding anoth­er stack frame when using caller or Carp.

With all that in mind, here’s the result:

sub old_method {
    warn 'old_method is deprecated';
    no warnings 'redefine';
    *old_method = \&new_method;
    goto &new_method;

sub new_method {
    # code from old_method goes here

Old (and not-​so-​old) hands at pro­gram­ming are prob­a­bly leap­ing out of their seats right now yelling, YOU’RE USING GOTO! GOTO IS CONSIDERED HARMFUL!” And they’re right, but this isn’t Dijkstra’s goto. From the Perl manual:

The goto &NAME form is quite dif­fer­ent from the oth­er forms of goto. In fact, it isn’t a goto in the nor­mal sense at all, and does­n’t have the stig­ma asso­ci­at­ed with oth­er gotos. Instead, it exits the cur­rent sub­rou­tine (los­ing any changes set by local) and imme­di­ate­ly calls in its place the named sub­rou­tine using the cur­rent val­ue of @_. […] After the goto, not even caller will be able to tell that this rou­tine was called first.

perl­func man­u­al page

Computer sci­en­tists call this tail call elim­i­na­tion. The bot­tom line is that this achieves our third goal above: imme­di­ate­ly jump­ing to the new method as if it were orig­i­nal­ly called.

The oth­er tricky bit is in the line before, when we’re redefin­ing old_method to point to new_method while we’re still inside old_method. (Yes, you can do this.) If you’re run­ning under use warnings (and we are, and you should), you first need to dis­able that warn­ing. Later calls to old_method will go straight to new_method with­out log­ging anything.

And that’s it. The next step after launch­ing this change is to add a sto­ry to our back­log to mon­i­tor our logs for calls to the old method, and grad­u­al­ly refac­tor our oth­er repos­i­to­ries. Then we can final­ly remove the old method wrapper.