PERLPOLICY(1) Perl Programmers Reference Guide PERLPOLICY(1)
NAME
perlpolicy - Various and sundry policies and commitments related to the Perl core
DESCRIPTION
This document is the master document which records all written policies about how the Perl 5 Porters
collectively develop and maintain the Perl core.
GOVERNANCE
Perl 5 Porters
Subscribers to perl5-porters (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 any
of the Perl programming languages. These days, Larry spends most of his time on Perl 6, while Perl 5 is
shepherded by a "pumpking", a porter responsible for deciding what goes into each release and ensuring that
releases happen on a regular basis.
Larry sees Perl development along the lines of the US government: there's the Legislature (the porters), the
Executive branch (the -pumpking), 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:
1. Larry is always by definition right about how Perl should behave. This means he has final veto power on
the core functionality.
2. 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.
MAINTENANCE AND SUPPORT
Perl 5 is developed by a community, not a corporate entity. Every change contributed to the Perl core is the
result of a donation. Typically, these donations are contributions of code or time by individual members of
our community. On occasion, these donations come in the form of corporate or organizational sponsorship of a
particular individual or project.
As a volunteer organization, the commitments we make are heavily dependent on the goodwill and hard work of
individuals who have no obligation to contribute to Perl.
That being said, we value Perl's stability and security and have long had an unwritten covenant with the
broader Perl community to support and maintain releases of Perl.
This document codifies the support and maintenance commitments that the Perl community should expect from
Perl's developers:
· We "officially" support the two most recent stable release series. 5.12.x and earlier are now out of
· We encourage vendors to ship the most recent supported release of Perl at the time of their code freeze.
· As a vendor, you may have a requirement to backport security fixes beyond our 3 year support commitment.
We can provide limited support and advice to you as you do so and, where possible will try to apply those
patches to the relevant -maint branches in git, though we may or may not choose to make numbered releases
or "official" patches available. Contact us at <[email protected]> to begin that process.
BACKWARD COMPATIBILITY AND DEPRECATION
Our community has a long-held belief that backward-compatibility is a virtue, even when the functionality in
question is a design flaw.
We would all love to unmake some mistakes we've made over the past decades. Living with every design error
we've ever made can lead to painful stagnation. Unwinding our mistakes is very, very difficult. Doing so
without actively harming our users is nearly impossible.
Lately, ignoring or actively opposing compatibility with earlier versions of Perl has come into vogue.
Sometimes, a change is proposed which wants to usurp syntax which previously had another meaning. Sometimes,
a change wants to improve previously-crazy semantics.
Down this road lies madness.
Requiring end-user programmers to change just a few language constructs, even language constructs which no
well-educated developer would ever intentionally use is tantamount to saying "you should not upgrade to a new
release of Perl unless you have 100% test coverage and can do a full manual audit of your codebase." If we
were to have tools capable of reliably upgrading Perl source code from one version of Perl to another, this
concern could be significantly mitigated.
We want to ensure that Perl continues to grow and flourish in the coming years and decades, but not at the
expense of our user community.
Existing syntax and semantics should only be marked for destruction in very limited circumstances. If a given
language feature's continued inclusion in the language will cause significant harm to the language or prevent
us from making needed changes to the runtime, then it may be considered for deprecation.
Any language change which breaks backward-compatibility should be able to be enabled or disabled lexically.
Unless code at a given scope declares that it wants the new behavior, that new behavior should be disabled.
Which backward-incompatible changes are controlled implicitly by a 'use v5.x.y' is a decision which should be
made by the pumpking in consultation with the community.
When a backward-incompatible change can't be toggled lexically, the decision to change the language must be
considered very, very carefully. If it's possible to move the old syntax or semantics out of the core
language and into XS-land, that XS module should be enabled by default unless the user declares that they want
a newer revision of Perl.
Historically, we've held ourselves to a far higher standard than backward-compatibility -- bugward-
compatibility. Any accident of implementation or unintentional side-effect of running some bit of code has
been considered to be a feature of the language to be defended with the same zeal as any other feature or
functionality. No matter how frustrating these unintentional features may be to us as we continue to improve
Perl, these unintentional features often deserve our protection. It is very important that existing software
written in Perl continue to work correctly. If end-user developers have adopted a bug as a feature, we need
to treat it as such.
New syntax and semantics which don't break existing language constructs and syntax have a much lower bar.
deprecated
If something in the Perl core is marked as deprecated, we may remove it from the core in the next stable
release series, though we may not. As of Perl 5.12, deprecated features and modules warn the user as
they're used. If you use a deprecated feature and believe that its removal from the Perl core would be a
mistake, please contact the perl5-porters mailinglist and plead your case. We don't deprecate things
without a good reason, but sometimes there's a counterargument we haven't considered. Historically, we
did not distinguish between "deprecated" and "discouraged" features.
discouraged
From time to time, we may mark language constructs and features which we consider to have been mistakes as
discouraged. Discouraged features aren't candidates for removal in the next major release series, but we
may later deprecate them if they're found to stand in the way of a significant improvement to the Perl
core.
removed
Once a feature, construct or module has been marked as deprecated for a stable release cycle, we may
remove it from the Perl core. Unsurprisingly, we say we've removed these things.
MAINTENANCE BRANCHES
· New releases of maint should contain as few changes as possible. If there is any question about whether a
given patch might merit inclusion in a maint release, then it almost certainly should not be included.
· Portability fixes, such as changes to Configure and the files in hints/ are acceptable. Ports of Perl to a
new platform, architecture or OS release that involve changes to the implementation are NOT acceptable.
· Acceptable documentation updates are those that correct factual errors, explain significant bugs or
deficiencies in the current implementation, or fix broken markup.
· Patches that add new warnings or errors or deprecate features are not acceptable.
· Patches that fix crashing bugs that do not otherwise change Perl's functionality or negatively impact
performance are acceptable.
· Patches that fix CVEs or security issues are acceptable, but should be run through the
[email protected] mailing list rather than applied directly.
· Patches that fix regressions in perl's behavior relative to previous releases are acceptable.
· Updates to dual-life modules should consist of minimal patches to fix crashing or security issues (as
above).
· Minimal patches that fix platform-specific test failures or installation issues are acceptable. When these
changes are made to dual-life modules for which CPAN is canonical, any changes should be coordinated with
the upstream author.
· New versions of dual-life modules should NOT be imported into maint. Those belong in the next stable
series.
· Patches that add or remove features are not acceptable.
· Patches that break binary compatibility are not acceptable. (Please talk to a pumpking.)
Getting changes into a maint branch
Historically, only the pumpking cherry-picked changes from bleadperl into maintperl. This has...scaling
control over their work, and that it is a responsibility of the rest of the Perl community to ensure that they
retain this control. It is an attempt to document the standards to which we, as Perl developers, intend to
hold ourselves. It is an attempt to write down rough guidelines about the respect we owe each other as Perl
developers.
This statement is not a legal contract. This statement is not a legal document in any way, shape, or form.
Perl is distributed under the GNU Public License and under the Artistic License; those are the precise legal
terms. This statement isn't about the law or licenses. It's about community, mutual respect, trust, and
good-faith cooperation.
We recognize that the Perl core, defined as the software distributed with the heart of Perl itself, is a joint
project on the part of all of us. From time to time, a script, module, or set of modules (hereafter referred
to simply as a "module") will prove so widely useful and/or so integral to the correct functioning of Perl
itself that it should be distributed with the Perl core. This should never be done without the author's
explicit consent, and a clear recognition on all parts that this means the module is being distributed under
the same terms as Perl itself. A module author should realize that inclusion of a module into the Perl core
will necessarily mean some loss of control over it, since changes may occasionally have to be made on short
notice or for consistency with the rest of Perl.
Once a module has been included in the Perl core, however, everyone involved in maintaining Perl should be
aware that the module is still the property of the original author unless the original author explicitly gives
up their ownership of it. In particular:
· The version of the module in the Perl core should still be considered the work of the original author.
All patches, bug reports, and so forth should be fed back to them. Their development directions should be
respected whenever possible.
· Patches may be applied by the pumpkin holder without the explicit cooperation of the module author if and
only if they are very minor, time-critical in some fashion (such as urgent security fixes), or if the
module author cannot be reached. Those patches must still be given back to the author when possible, and
if the author decides on an alternate fix in their version, that fix should be strongly preferred unless
there is a serious problem with it. Any changes not endorsed by the author should be marked as such, and
the contributor of the change acknowledged.
· The version of the module distributed with Perl should, whenever possible, be the latest version of the
module as distributed by the author (the latest non-beta version in the case of public Perl releases),
although the pumpkin holder may hold off on upgrading the version of the module distributed with Perl to
the latest version until the latest version has had sufficient testing.
In other words, the author of a module should be considered to have final say on modifications to their module
whenever possible (bearing in mind that it's expected that everyone involved will work together and arrive at
reasonable compromises when there are disagreements).
As a last resort, however:
If the author's vision of the future of their module is sufficiently different from the vision of the pumpkin
holder and perl5-porters as a whole so as to cause serious problems for Perl, the pumpkin holder may choose to
formally fork the version of the module in the Perl core from the one maintained by the author. This should
not be done lightly and should always if at all possible be done only after direct input from Larry. If this
is done, it must then be made explicit in the module as distributed with the Perl core that it is a forked
version and that while it is based on the original author's work, it is no longer maintained by them. This
must be noted in both the documentation and in the comments in the source of the module.
Again, this should be a last resort only. Ideally, this should never happen, and every possible effort at
the health of the community and Perl itself. Members of a community should not normally have to resort to
rules and laws to deal with each other, and this document, although it contains rules so as to be clear, is
about an attitude and general approach. The first step in any dispute should be open communication, respect
for opposing views, and an attempt at a compromise. In nearly every circumstance nothing more will be
necessary, and certainly no more drastic measure should be used until every avenue of communication and
discussion has failed.
DOCUMENTATION
Perl's documentation is an important resource for our users. It's incredibly important for Perl's
documentation to be reasonably coherent and to accurately reflect the current implementation.
Just as P5P collectively maintains the codebase, we collectively maintain the documentation. Writing a
particular bit of documentation doesn't give an author control of the future of that documentation. At the
same time, just as source code changes should match the style of their surrounding blocks, so should
documentation changes.
Examples in documentation should be illustrative of the concept they're explaining. Sometimes, the best way
to show how a language feature works is with a small program the reader can run without modification. More
often, examples will consist of a snippet of code containing only the "important" bits. The definition of
"important" varies from snippet to snippet. Sometimes it's important to declare "use strict" and "use
warnings", initialize all variables and fully catch every error condition. More often than not, though, those
things obscure the lesson the example was intended to teach.
As Perl is developed by a global team of volunteers, our documentation often contains spellings which look
funny to somebody. Choice of American/British/Other spellings is left as an exercise for the author of each
bit of documentation. When patching documentation, try to emulate the documentation around you, rather than
changing the existing prose.
In general, documentation should describe what Perl does "now" rather than what it used to do. It's perfectly
reasonable to include notes in documentation about how behaviour has changed from previous releases, but, with
very few exceptions, documentation isn't "dual-life" -- it doesn't need to fully describe how all old versions
used to work.
CREDITS
"Social Contract about Contributed Modules" originally by Russ Allbery <[email protected]> and the
perl5-porters.
perl v5.16.3 2013-03-04 PERLPOLICY(1)
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