Pan T. Hose's Journal: Larry Wall's Apocalypse 12: OO in Perl 6

The long awaited Apocalypse 12 by Larry Wall has been just announced by chromatic on perl6-language mailing list. It is one of the most important documents explaining the Perl 6 language design. (All of the previous design decisions are available as Apocalypses by Larry Wall, Exegeses by Damian Conway and Synopses by Luke Palmer, Damian Conway and Allison Randal.) Apocalypse 12 talks about Object Oriented aspects of Perl 6, i.e. about Objects, Classes, Roles (also known as Traits), Multiple Dispatch and also covers some non-OO decisions:

"The official, unofficial slogan of Perl 6 is "Second System Syndrome Done Right!". After you read this Apocalypse you will at least be certain that we got the "Second System" part down pat. But we've also put in a little bit of work on the "Done Right" part, which we hope you'll recognize. The management of complexity is complex, but only if you think about it. The goal of Perl 6 is to discourage you from thinking about it unnecessarily." --- Larry Wall.

Table of contents:

Some of the Problems with Perl 5 OO

• A little too orthogonal
• Not quite orthogonal enough
• Forced non-encapsulation
• A little too minimal
• Too much keyword reuse
• Too difficult to capture metadata
• Inside-out interfaces
• Not enough convention
• Wrong conventions
• Everything possible, but difficult

Perl 5 Non-Problems

• Generating class by running code at compile time
• There are many roads to polymorphism
• People using a class shouldn't have to think hard

Trust in Convention, But Keep Your Powder Dry

An Easy Example

Classes

• Declaration of classes
  • Class traits
• Use of classes
• Class name semantics
• Private classes

Methods

• Declaration of methods
• Use of methods
  • The dot notation
  • The "indirect object" notation
  • Calling private methods

Class Methods

• Class invocant
• Class|object invocant

Submethods

Attributes

• Declaration of attributes
• Use of attributes
• Default values

Class Attributes

Object Construction

• The default constructor
  • The default cloner
• CREATE
• BUILDALL
• BUILD
• Eliminating redundancy in constructor calls

Object Deconstruction

• DESTROYALL
• DESTROY

Dispatch Mechanisms

• Single dispatch
• Calling superclasses, and not-so-superclasses
• Parallel dispatch
• WALKCLASS and WALKMETH caching

Multiple Dispatch

• Declaration of multiple dispatch routines
  • multi sub
  • multi sub * (tradition multimethods)
  • multi method
  • multi submethod
  • multi rule
  • multi submethod BUILD
  • multi method constructors
• Calling via multiple dispatch
  • Multiple dispatch semantics
  • Single dispatch semantics
  • Indirect objects
• Meaning of "next METHOD"
• Making fiends, er, friends

Overloading

• Binary ops

Class Composition with Roles

• Declaration of roles
  • Parametric types
  • Interfaces
  • Private interfaces
  • Encapsulated attributes
  • Encapsulated private attributes
  • Encapsulated delegation
  • Encapsulated inheritance
• Use of roles at compile time
  • Conflict resolution
• Use of roles at runtime (mixins)

Traits

Delegation

• Wildcard delegation
• Renaming delegated methods
• Delegation without an attribute
• Delegation of handlers
• Hash-based redispatch
• Relationship to roles
• Anonymous delegation for ISA emulation

Types and Subtypes

Enums

Open vs. Closed Classes

Interface Consistency

Collections of Classes

• In classes
• In modules

Versioning

• Version declarations
• Use of version and author wildcards

Introspection

Other Non-OO Decisions

• Exportation
• The gather/take construct
• :foo() adverbs
• Special quoting of identifiers inside curlies going away
• Vector operators renamed back to "hyper" operators
• Unary hyper operators now use one quote rather than two
• $thumb.twiddle no longer requires parens when interpolated
• The =:= identity operator
• New grammatical categories
• Assignment to state variable declaration now does "first" semantics
• The length() function is gone
• String positions
• The new "&" separator in Regexen

Optional Mandatory Cross-Disciplinary Joke for People Tired of Dogs

Future Directions

References...er, Reference...

You can access the entire document as a print friendly version. The standard version of Apocalypse 12 is divided into 20 parts. Enjoy.

If you are new to Perl 6 and Parrot, then Perl 6 Essentials by Allison Randal, Dan Sugalski and Leopold Tötsch might be a great introduction. The second edition should be published soon.

Slashdot story submitted on 11:14 Saturday 17 April 2004 UTC. Pending...

Rejected

[Pan T. Hose]

Surprisingly enough, the story has been rejected. I wonder why... There is nothing posted about the Apocalypse 12 yet. Very strange. Maybe I will try to repost it without the entire table of contents. It is just too important to be left unmentioned on Slashdot. I cannot let it happen.

Reposted

[Pan T. Hose]

Update: Some other story about Apocalypse 12 has been already posted. It is not very informative and lacks lots of extremely important links and background, so I have partially resubmitted my rejected story as a first post [slashdot.org] instead. I hope it will help people learn more about the most innovative and cutting-edge programing language in existance, which Perl 6 [perl.org] is soon going to be. I would hereby also like to thank Larry Wall for the absolutely amazing work he is doing. I hope he reads my journal. Thanks Larry!

Rejected ...

[osewa77]

I haven't posted the story before. maybe their were several submissions and they just had to choo0se one. cheers ;)

Very Disappointing

[Pan T. Hose]

I haven't posted the story before. maybe their were several submissions and they just had to choo0se one. cheers ;)

There must have been several submissions indeed and they have apparently chosen one [slashdot.org] which was not only posted five hours later but was also in every way inferior to mine [slashdot.org]. The reasons of that decision remain unclear. But even though my story has been so foolishly rejected, I still managed to post three out of four Score:5 comments including (but not limited to) one First Post [slashdot.org] while getti