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Perl 6 and Parrot Essentials, Second Edition by Leopold Tötsch, Dan Sugalski, Allison Randal

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The Continuing Mission

Much has changed since the early days of the project. New people join and others leave in a regular “changing of the guard” pattern. Plans change as the work progresses, and the demands of the work and the needs of the community become clearer. Today the Perl 6 project has two major parts: language design and internals. Each branch is relatively autonomous, though there is a healthy amount of coordination between them.

Language Design

As with all things Perl, the central command of the language design process is Larry Wall, the creator of the Perl language. Larry is supported by the rest of the design team: Damian Conway, Allison Randal, Dan Sugalski, Hugo van der Sanden, and chromatic. We speak in weekly teleconferences and also meet face-to-face a few times a year to hash out ideas for the design documents, or to work through roadblocks standing in the way of design or implementation. The design team is a diverse group, including programmers-for-hire, Perl trainers, and linguists with a broad spectrum of interests and experiences. This diversity has proved quite valuable in the design process, as each member is able to see problems in the design or potential solutions that the other members missed.

Requests For Comments (RFCs)

The first step in designing the new language was the RFC (Request For Comments) process. This spurred an initial burst of community involvement. Anyone was free to submit an RFC on any subject, whether it was as small as adding an operator, or as big as reworking OO syntax. Most of the proposals were really quite conservative. The RFCs followed a standard format so they would be easier to read and easier to compare.

Each RFC was subject to peer review, carried out in an intense few weeks around October 2000. One thing the RFC process demonstrated was that the Perl community still wasn’t quite ready to move beyond the infighting that had characterized Perl 5 Porters earlier that year.[1] Even though few RFCs have been accepted without modification, the process identified a large number of irritants in the language. These have served as signposts for later design efforts.

Apocalypses, Synopses, Exegeses

The Apocalypses,[2] Synopses, and Exegeses[3] are an important part of the design process. Larry started the Apocalypse series as a systematic way of answering the RFCs. Each Apocalypse corresponds to a chapter in his book Programming Perl, and addresses the features in the chapter that are likely to change.

However, the Apocalypses have become much more than a simple response to RFCs. Larry has a startling knack for looking at 12 solutions to a problem, pulling out the good bits from each one, and combining them into a solution that is 10 times better than any of the proposals alone. The Apocalypses are an excellent example of this “Larry Effect.” He addresses each relevant RFC, and gives reasons why he accepted or rejected various pieces of it. But each Apocalypse also goes beyond a simple “yes” and “no” response to attack the roots of the problems identified in the RFCs.

The Synopses are summaries of each Apocalypse. These act as a quick reference for the current state of design, and are more approachable than the often lengthy Apocalypses. The Synopsis series didn’t start until Apocalypse 5, but Luke Palmer is now working on the retroactive Synopses 2-4.

Damian Conway’s Exegeses are extensions of each Apocalypse. The Exegeses are built around practical code examples that apply and explain the new ideas.

The p6l mailing list

The next body of design work is the Perl 6 Language mailing list (), often fondly referred to as " p6l.” Piers Cawley writes a weekly summary of all the Perl 6 mailing lists. Luke Palmer has been deputized as unofficial referee of the list. He answers questions that don’t require the direct involvement of the design team or that have been answered before. The list has approximately 40 regular contributors in any given month, as well as a large number of occasional posters and lurkers. Some people have participated since the very beginning; others appear for a few months and move on.

Even though the individuals change, the general tone of p6l is the same. It’s an open forum for any ideas on the user-visible parts of Perl 6. In the typical pattern, one person posts an idea and 5 to 10 people respond with criticisms or suggestions. The list periodically travels down a speculative thread like a runaway train, but these eventually run out of steam. Then Larry picks out the golden bits and gently tells the rest that no, he never intended Perl 6 to have neo-Vulcan mechanoid Scooby-Dooby-doos. Even when Larry doesn’t post, he follows the list and the traffic serves as a valuable catalyst for his thoughts.

Internals

The internals development for Perl 6 falls to the Parrot project. The heart of Parrot is a grandiose idea that turned out to be more realistic than anyone originally could have believed: why not have a single interpreter for several languages? Unlike the parent Perl 6 project, which was launched in a single day, the plan for Parrot formed in bits and pieces over the period of a year.

On April 1, 2001, Simon Cozens published an article titled “Programming Parrot” as an April Fools’ joke (http://www.perl.com/pub/a/2001/04/01/parrot.htm). It was a contrived interview with Larry Wall and Guido van Rossum detailing their plans to merge Python and Perl into a new language called Parrot. A few months later, when Perl 6 internals began to take an independent path within the larger project, they dubbed the subproject “Parrot” in a fitting turn of life imitating art.

The plan for Parrot was to build a language-neutral run-time environment. It would support all the features of dynamic languages, such as Python, Ruby, Scheme, Befunge, and others. It would have threading and Unicode support (two of the most problematic features to add into Perl 5 code) designed in from the start. It would support exceptions and compilation to bytecode, and have clean extension and embedding mechanisms.

The language-neutral interpreter was originally just a side effect of good design. Keeping the implementation independent of the syntax would make the code cleaner and easier to maintain. One practical advantage of this design was that Parrot development could begin even though the Perl 6 language specification was still in flux.

The bigger win in the long term, though, was that since Parrot would support the features of the major dynamic languages and wasn’t biased to a particular syntax, it could run all these languages with little additional effort. It’s generally acknowledged that different languages are suited to different tasks. Picking which language will be used in a large software project is a common planning problem. There’s never a perfect fit. It usually boils down to picking the language with the most advantages and the least noticeable disadvantages. The ability to easily combine multiple languages within a project could be a huge benefit. Use well-tested libraries from one language for one task. Take advantage of a clean way of expressing a particular problem domain in a second, without being forced to use it in areas where it’s weak.

The modular design also benefits future language designers. Instead of targeting lex/yacc and reimplementing low-level features such as garbage collection and dynamic types, designers can write a parser that targets the Parrot virtual machine.

Dan Sugalski leads the Parrot project as chief architect, and Leopold Tötsch is the current pumpking. The Parrot project is largely autonomous. Dan coordinates with the rest of the design team to ensure that Parrot will be able to support the semantics Perl 6 will require, but the language designers have very little input into the details of implementation. Parrot isn’t developed solely for Perl, but Perl 6 is entirely dependent on Parrot—it is the only interpreter for Perl 6.

The core communication line for the Parrot project is the mailing list, , otherwise known as " p6i.” It’s a much more business-like list than p6l. Workflow in Parrot takes the form of submitted patches. Anyone is free to submit a patch, and contributors who consistently submit valuable patches over a long period of time are granted check-in access to the CVS repository.

Ponie

Ponie is an implementation of Perl 5 on Parrot, started in July 2003. Offically, Ponie stands for " Perl On New Internal Engine.” The name was originally derived from a running gag in the London.pm Perl Mongers group where the phrase “I want a pony” appeared in lists of feature requests for Perl (and other unusual places).

The project, led by Artur Bergman, has taken the Perl 5 source code as a base and is gradually replacing the core elements with Parrot equivalents. Legacy code will be one of the biggest obstacles to projects considering the move from Perl 5 to Perl 6. Few companies have the resources to do a complete update to existing code every time a new version of the language is released. Ponie offers a smooth migration path that ensures Perl 5 code will function as long as it’s needed. You’ll even be able to use Perl 5 modules and Perl 6 modules side-by-side in the same program. The current plan is for Ponie to be the 5.14 or 5.16 release of Perl.

The mailing list for Ponie development is .

Supporting Structure

Last, but not least, is the glue that holds the project together. Ask Bjørn Hansen and Robert Spier manage the email, revision control, and bug-tracking systems, as well as the web pages for Perl 6, Parrot, and Ponie (http://dev.perl.org). Without these systems, the project would grind to a screeching halt.

Allison Randal is the project manager. As is typical of open source development projects, managing the Perl 6 project is quite different from managing a commercial project of the same size and complexity. There are no schedules, no deadlines, no hiring and firing, and no salaries, bonuses, or stock options. There are no employees or bosses; there is very little hierarchy whatsoever. Management in this context isn’t about giving orders, it’s about making sure everyone has what they need to keep moving forward.

In the end, it is the developers themselves who hold the project together. Individuals bear their own share of responsibility for finding tasks that suit their skills, coordinating with others to keep duplicated effort minimal, and making sure the job gets done.



[1] Mark-Jason Dominus wrote an excellent critique of the RFC process (http://www.perl.com/pub/a/2000/11/perl6rfc.html). It may seem harsh to people accustomed to the more open and tolerant community of today, but it’s an accurate representation of the time when it was written.

[2] An “apocalypse” in the sense of “revelation,” not “end of the world.”

[3] An “exegesis” is an explanation or interpretation of a text.

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