perlhack perlhack is a Perl module that will show you how to hack at the Perl internals.
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perlhack is a Perl module that will show you how to hack at the Perl internals. perlhack is a Perl module that will show you how to hack at the Perl internals.This document attempts to explain how Perl development takes place, and ends with some suggestions for people wanting to become bona fide porters.The perl5-porters mailing list is where the Perl standard distribution is maintained and developed. The list can get anywhere from 10 to 150 messages a day, depending on the heatedness of the debate. Most days there are two or three patches, extensions, features, or bugs being discussed at a time.A searchable archive of the list is at either: http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/or http://archive.develooper.com/perl5-porters@perl.org/List subscribers (the porters themselves) come in several flavours. Some are quiet curious lurkers, who rarely pitch in and instead watch the ongoing development to ensure they're forewarned of new changes or features in Perl. Some are representatives of vendors, who are there to make sure that Perl continues to compile and work on their platforms. Some patch any reported bug that they know how to fix, some are actively patching their pet area (threads, Win32, the regexp engine), while others seem to do nothing but complain. In other words, it's your usual mix of technical people.Over this group of porters presides Larry Wall. He has the final word in what does and does not change in the Perl language. Various releases of Perl are shepherded by a "pumpking", a porter responsible for gathering patches, deciding on a patch-by-patch, feature-by-feature basis what will and will not go into the release. For instance, Gurusamy Sarathy was the pumpking for the 5.6 release of Perl, and Jarkko Hietaniemi was the pumpking for the 5.8 release, and Rafael Garcia-Suarez holds the pumpking crown for the 5.10 release.In addition, various people are pumpkings for different things. For instance, Andy Dougherty and Jarkko Hietaniemi did a grand job as the Configure pumpkin up till the 5.8 release. For the 5.10 release H.Merijn Brand took over.Larry sees Perl development along the lines of the US government: there's the Legislature (the porters), the Executive branch (the pumpkings), and the Supreme Court (Larry). The legislature can discuss and submit patches to the executive branch all they like, but the executive branch is free to veto them. Rarely, the Supreme Court will side with the executive branch over the legislature, or the legislature over the executive branch. Mostly, however, the legislature and the executive branch are supposed to get along and work out their differences without impeachment or court cases.You might sometimes see reference to Rule 1 and Rule 2. Larry's power as Supreme Court is expressed in The Rules:Larry is always by definition right about how Perl should behave. This means he has final veto power on the core functionality.Larry is allowed to change his mind about any matter at a later date, regardless of whether he previously invoked Rule 1.Got that? Larry is always right, even when he was wrong. It's rare to see either Rule exercised, but they are often alluded to.New features and extensions to the language are contentious, because the criteria used by the pumpkings, Larry, and other porters to decide which features should be implemented and incorporated are not codified in a few small design goals as with some other languages. Instead, the heuristics are flexible and often difficult to fathom. Here is one person's list, roughly in decreasing order of importance, of heuristics that new features have to be weighed against:Does concept match the general goals of Perl?These haven't been written anywhere in stone, but one approximation is: 1. Keep it fast, simple, and useful. 2. Keep features/concepts as orthogonal as possible. 3. No arbitrary limits (platforms, data sizes, cultures). 4. Keep it open and exciting to use/patch/advocate Perl everywhere. 5. Either assimilate new technologies, or build bridges to them.Where is the implementation?All the talk in the world is useless without an implementation. In almost every case, the person or people who argue for a new feature will be expected to be the ones who implement it. Porters capable of coding new features have their own agendas, and are not available to implement your (possibly good) idea.Backwards compatibilityIt's a cardinal sin to break existing Perl programs. New warnings are contentious--some say that a program that emits warnings is not broken, while others say it is. Adding keywords has the potential to break programs, changing the meaning of existing token sequences or functions might break programs.Could it be a module instead?Perl 5 has extension mechanisms, modules and XS, specifically to avoid the need to keep changing the Perl interpreter. You can write modules that export functions, you can give those functions prototypes so they can be called like built-in functions, you can even write XS code to mess with the runtime data structures of the Perl interpreter if you want to implement really complicated things. If it can be done in a module instead of in the core, it's highly unlikely to be added.Is the feature generic enough?Is this something that only the submitter wants added to the language, or would it be broadly useful? Sometimes, instead of adding a feature with a tight focus, the porters might decide to wait until someone implements the more generalized feature. For instance, instead of implementing a "delayed evaluation" feature, the porters are waiting for a macro system that would permit delayed evaluation and much more.Does it potentially introduce new bugs?Radical rewrites of large chunks of the Perl interpreter have the potential to introduce new bugs. The smaller and more localized the change, the better.Does it preclude other desirable features?A patch is likely to be rejected if it closes off future avenues of development. For instance, a patch that placed a true and final interpretation on prototypes is likely to be rejected because there are still options for the future of prototypes that haven't been addressed.Is the implementation robust?Good patches (tight code, complete, correct) stand more chance of going in. Sloppy or incorrect patches might be placed on the back burner until the pumpking has time to fix, or might be discarded altogether without further notice.Is the implementation generic enough to be portable?The worst patches make use of a system-specific features. It's highly unlikely that nonportable additions to the Perl language will be accepted.Is the implementation tested?Patches which change behaviour (fixing bugs or introducing new features) must include regression tests to verify that everything works as expected. Without tests provided by the original author, how can anyone else changing perl in the future be sure that they haven't unwittingly broken the behaviour the patch implements? And without tests, how can the patch's author be confident that his/her hard work put into the patch won't be accidentally thrown away by someone in the future?Is there enough documentation?Patches without documentation are probably ill-thought out or incomplete. Nothing can be added without documentation, so submitting a patch for the appropriate manpages as well as the source code is always a good idea.Is there another way to do it?Larry said "Although the Perl Slogan is There's More Than One Way to Do It, I hesitate to make 10 ways to do something". This is a tricky heuristic to navigate, though--one man's essential addition is another man's pointless cruft.Does it create too much work?Work for the pumpking, work for Perl programmers, work for module authors, ... Perl is supposed to be easy.Patches speak louder than wordsWorking code is always preferred to pie-in-the-sky ideas. A patch to add a feature stands a much higher chance of making it to the language than does a random feature request, no matter how fervently argued the request might be. This ties into "Will it be useful?", as the fact that someone took the time to make the patch demonstrates a strong desire for the feature.If you're on the list, you might hear the word "core" bandied around. It refers to the standard distribution. "Hacking on the core" means you're changing the C source code to the Perl interpreter. "A core module" is one that ships with Perl. Requirements: · Perl

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