woman in black tank top and blue denim jeans

This blog has devot­ed a fair amount of atten­tion to the pop­u­lar and mul­ti­fac­eted object-​oriented sys­tem Moose and its light­weight sub­set Moo. I’ve also cov­ered Object::Pad, the test­bed of con­cepts and syn­tax for Corinna, the pro­posed next-​generation Perl core OO sys­tem. But what if your project is too memory‑, performance‑, or dependency-​constrained for these options?

It turns out that CPAN has a rich his­to­ry of lighter-​weight OO mod­ules to meet many dif­fer­ent needs. If you can live with their trade-​offs, they’re worth inves­ti­gat­ing instead of rolling your own lay­er over Perl’s OO. Here are a few.

Class::Struct

Class::Structs main claim to fame is its inclu­sion in the stan­dard Perl dis­tri­b­u­tion, so there’s no need to install depen­den­cies from CPAN. It pro­vides a syn­tax for defin­ing class­es as C‑style structs at either com­pile time or run­time. (There’s no speed advan­tage to the for­mer; it just means that your class will be built as if you had writ­ten the acces­sors your­self as subs.) Here’s an example:

#!/usr/bin/env perl

use v5.24; # for strict, say, and postfix dereferencing
use warnings;

package Local::MyClass;
use Class::Struct (
    foo => '$',
    bar => '@',
    baz => '%',
);

package main;

my $obj = Local::MyClass->new(
    foo => 'hello',
    bar => [1, 2, 3],
    baz => { name => 'Mark'},
);

say $obj->foo, ' ', $obj->baz('name');
say join ',', $obj->bar->@*;

# replace the name element of baz
$obj->baz(name => 'Sharon');

# replace the second element of bar
$obj->bar(1, 'replaced');
say $obj->foo, ' ', $obj->baz('name');
say join ',', $obj->bar->@*;

And here’s the output:

hello Mark
1,2,3
hello Sharon
1,replaced,3

Note that Class::Struct sup­ports acces­sors for scalar, array, and hash types, as well as oth­er class­es (not demon­strat­ed). Consult the module’s doc­u­men­ta­tion for the dif­fer­ent ways to define and retrieve them.

Class::Accessor

Class::Accessor does one thing: it makes acces­sors and muta­tors (also known as get­ters and set­ters) for fields in your class. Okay, it actu­al­ly does anoth­er thing: it pro­vides your class with a new method to ini­tial­ize those fields. Those acces­sors can be read-​write, read-​only, or write-​only. (Why would you want write-​only acces­sors?) You can define any of them using either its his­tor­i­cal class meth­ods or a Moose-​like attribute syn­tax.

If you’re try­ing to squeeze every bit of per­for­mance out of your code and can sac­ri­fice a lit­tle flex­i­bil­i­ty in alter­ing acces­sor behav­ior, you can opt for Class::Accessor::Fast or Class::Accessor::Faster. The for­mer still uses hash ref­er­ences under the hood to rep­re­sent objects and the lat­ter uses array ref­er­ences. The main Class::Accessor doc­u­men­ta­tion con­tains an effi­cien­cy com­par­i­son of the three for your edification.

Here’s an exam­ple script using Class::Accessor::Faster and the Moose-​like syntax:

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyClass;
use Class::Accessor::Faster 'moose-like';

has readwrite => (is => 'rw');
has readonly  => (is => 'ro');

package main;

my $obj = Local::MyClass->new( { # must be a hash reference
    readwrite => 'hello',
    readonly  => 'world',
} );

say $obj->readwrite, ' ', $obj->readonly;
$obj->readwrite('greetings');
say $obj->readwrite, ' ', $obj->readonly;

# throws an error
$obj->readonly('Cleveland');

And here is its output:

hello world
greetings world
'main' cannot alter the value of 'readonly' on objects of class 'Local::MyClass' at ./caf.pl line 24.

Class::Tiny

Class::Tiny both does less and more than Class::Accessor. All of its gen­er­at­ed acces­sors are read-​write, but you can also give their attrib­ut­es lazy defaults. Its gen­er­at­ed con­struc­tor takes argu­ments via either a Class::Accessor-style hash ref­er­ence or a plain list of key/​value pairs, so that’s a lit­tle more con­ve­nient. It also sup­ports Moose-​style BUILDARGS, BUILD, and DEMOLISH meth­ods for argu­ment adjust­ment, val­i­da­tion, and object cleanup, respectively.

It’s a toss-​up as to which of the pre­vi­ous two is bet­ter.” You’ll have to exam­ine their respec­tive fea­tures and deter­mine which ones map to your needs.

Here’s an exam­ple script that shows a few of Class::Tiny’s unique features:

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyClass;
use Class::Tiny qw<foo bar>,
{
    baz       => 'default baz',
    timestamp => sub { time },
};

package main;

my $obj = Local::MyClass->new( # plain key-values OK
    foo => 'hello',
    bar => 'world',
);

say $obj->foo, ' ', $obj->bar;
say 'Object built on ', scalar localtime $obj->timestamp;
$obj->foo('greetings');
$obj->bar('Cleveland');
say $obj->foo, ' ', $obj->bar;
say $obj->baz;

And its output:

hello world
Object built on Tue Sep  7 09:00:00 2021
greetings Cleveland
default baz

Object::Tiny

For an even more min­i­mal­ist approach, con­sid­er Object::Tiny. Its acces­sors are read-​only, it gives you a sim­ple con­struc­tor, and that’s it. Its doc­u­men­ta­tion lists a num­ber of rea­sons why it can be supe­ri­or to Class::Accessor, includ­ing low­er mem­o­ry usage and less typ­ing. There’s also a fork called Object::Tiny::RW that adds read-​write sup­port to its accessors.

Class::Tiny’s doc­u­men­ta­tion con­tains a fea­ture table com­par­i­son of it, Object::Tiny, and Class::Accessor. This may help you decide which to use.

Here’s an exam­ple script:

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyClass;
use Object::Tiny qw<foo bar>;

package main;

my $obj = Local::MyClass->new(
    foo => 'hello',
    bar => 'world',
);

say $obj->foo, ' ', $obj->bar;

# has no effect unless you use Object::Tiny::RW
$obj->foo('greetings');
say $obj->foo, ' ', $obj->bar;

And its output:

hello world
hello world

Add some speed with XS

If the above options are still too slow and you don’t mind requir­ing a C com­pil­er to install them, there are vari­ants that use Perl’s XS inter­face instead of pure Perl code:

Roles with Role::Tiny

If you’re eye­ing Moose and Moo’s sup­port for roles (also known as traits) as an alter­na­tive to inher­i­tance but still want to keep things light with one of the above mod­ules, you’re in luck. The Role::Tiny mod­ule lets you com­pose meth­ods into con­sum­ing class­es with Moo-​like syn­tax and will pull in Common Lisp Object System-style method mod­i­fi­er sup­port from Class::Method::Modifiers if you need it. It does mean anoth­er cou­ple of CPAN depen­den­cies, so if that’s a prob­lem in your sit­u­a­tion you’ll just have to live with­out roles.

Here’s an exam­ple script with a role and a con­sum­ing class that uses Class::Tiny. The role requires that its con­sumers imple­ment a required_method, pro­vides a foo method that uses it, and a method mod­i­fi­er for bar.

#!/usr/bin/env perl

use v5.12; # for strict and say
use warnings;

package Local::MyRole;
use Role::Tiny;

requires 'required_method';

sub foo {
    my $self = shift;
    say $self->required_method();
}

before bar => sub {
    warn 'About to call bar...';
};

package Local::MyClass;
use Class::Tiny {name => ''};
use Role::Tiny::With;
with 'Local::MyRole';

sub bar {
    my ($self, $greeting) = @_;
    say "$greeting ", $self->name;
}

sub required_method {
    my $self = shift;
    return 'Required by Local::MyRole';
}

package main;

my $obj = Local::MyClass->new(name => 'Mark');
$obj->bar('hello');

$obj->name('Sharon');
$obj->bar('salutations');

$obj->foo();

And its output:

About to call bar... at ./rt.pl line 17.
hello Mark
About to call bar... at ./rt.pl line 17.
salutations Sharon
Required by Local::MyRole

What’s your favorite?

There will always be those who insist on writ­ing every­thing long­hand, but mod­ules like these can save a lot of time and typ­ing as well as reduce errors. Do you have a favorite, maybe some­thing I missed? Let me know in the comments.

close up of gear shift over black background

Last week found me explor­ing Object::Pad as an alter­na­tive to the Moo object-​oriented frame­work for Perl since the for­mer is pro­to­typ­ing the syn­tax and con­cepts for a pro­posed built-​in OO frame­work named Corinna. I had to put that par­tic­u­lar project on hold as dbcrit­ics cur­rent design is a bit too role-​happy and Object::Pad cur­rent­ly lacks method mod­i­fiers as in Moo. (Corinna is explic­it­ly skip­ping them for its cur­rent min­i­mum viable prod­uct.) Thankfully, devel­op­ment con­tin­ues at a rapid pace. For instance, author Paul Evans has already addressed a prob­lem I ran into when attempt­ing to exam­ine slot val­ues in the debugger.

But I want­ed to high­light a point I made in one of the com­ments last week: Object::Pad’s slots (a.k.a. fields, attrib­ut­es, what­ev­er) are pri­vate by default, com­plete­ly unex­posed to oth­er class instances unless they mon­key with the meta-​object pro­to­col. Unless you explic­it­ly define or gen­er­ate some kind of acces­sor method, these slots act like lex­i­cal (a.k.a. my) vari­ables and are only avail­able to meth­ods with­in the class.

Here’s an example:

use v5.14; # for say and package blocks
use Object::Pad 0.50;
use Feature::Compat::Try;

class Local::MyClass {
    has $arg           :param  = 'hello';
    has $readable_slot :reader = 'world';
    has $private_slot          = 'shh';

    method show_slots {
        say "You passed me $arg in the constructor.";
        say "I can see $readable_slot and you can use it as a reader.";
        say "Here's me using the reader too: ", $self->readable_slot;
        say "But only I can see $private_slot.";
        return;
    }
}

package main {
    my $obj = Local::MyClass->new(arg => 'foo');
    $obj->show_slots();
    say $obj->readable_slot;

    # Nope: Not a HASH reference
    try { say $obj->{private_slot} } catch ($e) { say "Nope: $e" }

    # Nope: Can't locate object method "private_slot" via package "Local::MyClass"
    try { say $obj->private_slot } catch ($e) { say "Nope: $e" }
}

This stands in stark con­trast to Perl’s more low-​tech hashref-​based objects, where all attrib­ut­es are avail­able sim­ply through deref­er­enc­ing the instance, e.g., $object->{foo}. Although dis­cour­aged, OO purists some­times ding Perl for this kind of unen­forced encap­su­la­tion, and I myself have seen code­bas­es that vio­late it despite the con­ven­tion of pre­ced­ing pri­vate method and attribute names with an under­score (_).

Unfortunately, there is not yet any way to declare an Object::Pad method pri­vate. You could use lex­i­cal sub­rou­tines, but then you lose the con­ve­nience of a pre-​made $self vari­able and acces­si­bil­i­ty through the MOP. The Corinna pro­pos­al lists sev­er­al dif­fer­ent types of meth­ods includ­ing pri­vate ones, so maybe this is an area for future Object::Pad development.

Another open ques­tion from the com­ments: How is [Object::Pad] on mem­o­ry and speed com­pared to Moo and blessed objects?” Luckily the pro­lif­ic per­lan­car has already added Object::Pad to his Bencher::Scenarios::Accessors dis­tri­b­u­tion, and from that, it appears that between it and Moo, Object::Pad is faster on start­up, neck-​and-​neck on object con­struc­tion and acces­sor gen­er­a­tion, and slow­er on reads and writes. (Note that Object::Pad is a fast-​moving tar­get so these fig­ures may not track with the lat­est ver­sion’s changes.) It’s no sur­prise that plain blessed objects fared bet­ter than both in most sce­nar­ios except for reads, where Moo was faster than hash-​based objects but slow­er than array-based.

I expect that should Corinna be built into Perl it would nar­row that gap with blessed objects, but in my mind, the advan­tages of using an object sys­tem out­weigh the per­for­mance hit 95% of the time. As far as bench­mark­ing mem­o­ry goes, I still need to test that on a Linux box (maybe my new VPS?) once I get more famil­iar with the Bencher framework.

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:

ADJUST {
    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;

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

has @violations;

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

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

Issues

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.

circus theme party

Last week’s arti­cle got a great response on Hacker News, and this par­tic­u­lar com­ment caught my eye:

I think this is the real point about Perl code read­abil­i­ty: it gives you enough flex­i­bil­i­ty to do things how­ev­er you like, and as a result many pro­gram­mers are faced with a mir­ror that reflects their own bad prac­tices back at them.

orev, Hacker News

This is why Damian Conway’s Perl Best Practices (2005) is one of my favorite books and perlcritic, the code ana­lyz­er is one of my favorite tools. (Though the for­mer could do with an update and the lat­ter includes poli­cies that con­tra­dict Conway.) Point perlcritic at your code, maybe add some oth­er poli­cies that agree with your house style, and grad­u­al­ly ratch­et up the sever­i­ty lev­el from gen­tle” to bru­tal.” All kinds of bad juju will come to light, from waste­ful­ly using grep to hav­ing too many sub­rou­tine argu­ments to catch­ing pri­vate vari­able use from oth­er pack­ages. perlcritic offers a use­ful base­line of con­duct and you can always cus­tomize its con­fig­u­ra­tion to your own tastes.

The oth­er con­for­mance tool in a Perl devel­op­er’s belt is perltidy, and it too has a Conway-​compatible con­fig­u­ra­tion as well as its default Perl Style Guide set­tings. I’ve found that more than any­thing else, perltidy helps set­tle argu­ments both between devel­op­ers and between their code in help­ing to avoid exces­sive merge conflicts.

But apart from extra tools, Perl the lan­guage itself can be bent and even bro­ken to suit just about any­one’s agen­da. Those used to more bondage-​and-​discipline lan­guages (hi, Java!) might feel revul­sion at the lengths to which this has some­times been tak­en, but per the quote above this is less an indict­ment of the lan­guage and more of its less method­i­cal pro­gram­mers.

Some of this behav­ior can be reha­bil­i­tat­ed with perlcritic and perltidy, but what about oth­er sins attrib­uted to Perl? Here are a few peren­ni­al favorites”:

Objects and Object-​Oriented Programming

Perl has a min­i­mal­ist object sys­tem based on earlier-​available lan­guage con­cepts like data struc­tures (often hash­es, which it has in com­mon with JavaScript), pack­ages, and sub­rou­tines. Since Perl 5’s release in 1994 much ver­bose OO code has been writ­ten using these tools.

The good news is that since 2007 we’ve had a sophis­ti­cat­ed metaobject-​protocol-​based lay­er on top of them called Moose, since 2010 a light­weight but forward-​compatible sys­tem called Moo, and a cou­ple of even tinier options as described in the Perl OO Tutorial. Waiting in the wings is Corinna, an effort to bring next-​generation object capa­bil­i­ties into the Perl core itself, and Object::Pad, a test­bed for some of the ideas in Corinna that you can use today in cur­rent code. (Really, please try it — the author needs feed­back!)

All this is to say that 99% of the time you nev­er need trou­ble your­self with bless, con­struc­tors, or writ­ing acces­sors for class or object attrib­ut­es. Smarter peo­ple than me have done the work for you, and you might even find a con­cept or three that you wish oth­er lan­guages had.

Contexts

There are two major ones: list and scalar. Another way to think of it is plur­al” vs. sin­gu­lar” in English, which is hope­ful­ly a thing you’re famil­iar with as you’re read­ing this blog.

Some func­tions in Perl act dif­fer­ent­ly depend­ing on whether the expect­ed return val­ue is a list or a scalar, and a func­tion will pro­vide a list or scalar con­text to its argu­ments. Mostly these act just as you would expect or would like them to, and you can find out how a func­tion behaves by read­ing its doc­u­men­ta­tion. Your own func­tions can behave like this too, but there’s usu­al­ly no need as both scalars and lists are auto­mat­i­cal­ly inter­pret­ed into lists.” Again, Perl’s DWIMmery at work.

Subroutine and Method Arguments

I’ve already writ­ten about this. Twice. And pre­sent­ed about it. Twice. The short ver­sion: Perl has sig­na­tures, but they’ve been con­sid­ered exper­i­men­tal for a while. In the mean­time, there are alter­na­tives on CPAN. You can even have type con­straints if you want.


I’ll leave you with this: Over the past month, Neil Bowers of the Perl Steering Council has been col­lect­ing quirks like these from Perl devel­op­ers. The PSC is review­ing this col­lec­tion for poten­tial doc­u­men­ta­tion fix­es, bug fix­es, fur­ther dis­cus­sion, etc. I would­n’t expect to see any fun­da­men­tal changes to the lan­guage out of this effort, but it’s a good sign that poten­tial­ly con­fus­ing fea­tures are being addressed.