collapse threading doc changes as suggested by Nicholas, adding in notes on the addit...

[p5sagit/p5-mst-13.2.git] / pod / perlfaq1.pod

=head1 NAME

perlfaq1 - General Questions About Perl

=head1 DESCRIPTION

This section of the FAQ answers very general, high-level questions
about Perl.

=head2 What is Perl?

Perl is a high-level programming language with an eclectic heritage
written by Larry Wall and a cast of thousands. It derives from the
ubiquitous C programming language and to a lesser extent from sed,
awk, the Unix shell, and at least a dozen other tools and languages.
Perl's process, file, and text manipulation facilities make it
particularly well-suited for tasks involving quick prototyping, system
utilities, software tools, system management tasks, database access,
graphical programming, networking, and world wide web programming.
These strengths make it especially popular with system administrators
and CGI script authors, but mathematicians, geneticists, journalists,
and even managers also use Perl. Maybe you should, too.

=head2 Who supports Perl? Who develops it? Why is it free?

The original culture of the pre-populist Internet and the deeply-held
beliefs of Perl's author, Larry Wall, gave rise to the free and open
distribution policy of perl. Perl is supported by its users. The
core, the standard Perl library, the optional modules, and the
documentation you're reading now were all written by volunteers. See
the personal note at the end of the README file in the perl source
distribution for more details. See L<perlhist> (new as of 5.005)
for Perl's milestone releases.

In particular, the core development team (known as the Perl Porters)
are a rag-tag band of highly altruistic individuals committed to
producing better software for free than you could hope to purchase for
money. You may snoop on pending developments via the archives at
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
and http://archive.develooper.com/perl5-porters@perl.org/
or the news gateway nntp://nntp.perl.org/perl.perl5.porters or
its web interface at http://nntp.perl.org/group/perl.perl5.porters ,
or read the faq at http://dev.perl.org/perl5/docs/p5p-faq.html ,
or you can subscribe to the mailing list by sending
perl5-porters-subscribe@perl.org a subscription request
(an empty message with no subject is fine).

While the GNU project includes Perl in its distributions, there's no
such thing as "GNU Perl". Perl is not produced nor maintained by the
Free Software Foundation. Perl's licensing terms are also more open
than GNU software's tend to be.

You can get commercial support of Perl if you wish, although for most
users the informal support will more than suffice. See the answer to
"Where can I buy a commercial version of perl?" for more information.

=head2 Which version of Perl should I use?

(contributed by brian d foy)

There is often a matter of opinion and taste, and there isn't any one
answer that fits everyone. In general, you want to use either the current
stable release, or the stable release immediately prior to that one.
Currently, those are perl5.10.x and perl5.8.x, respectively.

Beyond that, you have to consider several things and decide which is best
for you.

=over 4

=item *

If things aren't broken, upgrading perl may break them (or at least issue
new warnings).

=item *

The latest versions of perl have more bug fixes.

=item *

The Perl community is geared toward supporting the most recent releases,
so you'll have an easier time finding help for those.

=item *

Versions prior to perl5.004 had serious security problems with buffer
overflows, and in some cases have CERT advisories (for instance,
http://www.cert.org/advisories/CA-1997-17.html ).

=item *

The latest versions are probably the least deployed and widely tested, so
you may want to wait a few months after their release and see what
problems others have if you are risk averse.

=item *

The immediate, previous releases (i.e. perl5.8.x ) are usually maintained
for a while, although not at the same level as the current releases.

=item *

No one is actively supporting Perl 4. Five years ago it was a dead
camel carcass (according to this document). Now it's barely a skeleton
as its whitewashed bones have fractured or eroded.

=item *

There is no Perl 6 release scheduled, but it will be available when
it's ready. Stay tuned, but don't worry that you'll have to change
major versions of Perl; no one is going to take Perl 5 away from you.

=item *

There are really two tracks of perl development: a maintenance version
and an experimental version. The maintenance versions are stable, and
have an even number as the minor release (i.e. perl5.10.x, where 10 is the
minor release). The experimental versions may include features that
don't make it into the stable versions, and have an odd number as the
minor release (i.e. perl5.9.x, where 9 is the minor release).

=back


=head2 What are Perl 4, Perl 5, or Perl 6?

(contributed by brian d foy)

In short, Perl 4 is the past, Perl 5 is the present, and Perl 6 is the
future.

The number after perl (i.e. the 5 after Perl 5) is the major release
of the perl interpreter as well as the version of the language. Each
major version has significant differences that earlier versions cannot
support.

The current major release of Perl is Perl 5, and was released in 1994.
It can run scripts from the previous major release, Perl 4 (March 1991),
but has significant differences. It introduced the concept of references,
complex data structures, and modules. The Perl 5 interpreter was a
complete re-write of the previous perl sources.

Perl 6 is the next major version of Perl, but it's still in development
in both its syntax and design. The work started in 2002 and is still
ongoing. Many of the most interesting features have shown up in the
latest versions of Perl 5, and some Perl 5 modules allow you to use some
Perl 6 syntax in your programs. You can learn more about Perl 6 at
http://dev.perl.org/perl6/ .

See L<perlhist> for a history of Perl revisions.

=head2 What was Ponie?

(contributed by brian d foy)

Ponie stands for "Perl On the New Internal Engine", started by Arthur
Bergman from Fotango in 2003, and subsequently run as a project of The
Perl Foundation. It was abandoned in 2006
( http://www.nntp.perl.org/group/perl.ponie.dev/487 ).

Instead of using the current Perl internals, Ponie aimed to create a
new one that would provide a translation path from Perl 5 to Perl 6
(or anything else that targets Parrot, actually). You would have been
able to just keep using Perl 5 with Parrot, the virtual machine which
will compile and run Perl 6 bytecode.

=head2 What is Perl 6?

At The Second O'Reilly Open Source Software Convention, Larry Wall
announced Perl 6 development would begin in earnest. Perl 6 was an oft
used term for Chip Salzenberg's project to rewrite Perl in C++ named
Topaz. However, Topaz provided valuable insights to the next version
of Perl and its implementation, but was ultimately abandoned.

If you want to learn more about Perl 6, or have a desire to help in
the crusade to make Perl a better place then read the Perl 6 developers
page at http://dev.perl.org/perl6/ and get involved.

Perl 6 is not scheduled for release yet, and Perl 5 will still be supported
for quite awhile after its release. Do not wait for Perl 6 to do whatever
you need to do.

"We're really serious about reinventing everything that needs reinventing."
--Larry Wall

=head2 How stable is Perl?

Production releases, which incorporate bug fixes and new functionality,
are widely tested before release. Since the 5.000 release, we have
averaged only about one production release per year.

Larry and the Perl development team occasionally make changes to the
internal core of the language, but all possible efforts are made toward
backward compatibility. While not quite all Perl 4 scripts run flawlessly
under Perl 5, an update to perl should nearly never invalidate a program
written for an earlier version of perl (barring accidental bug fixes
and the rare new keyword).

=head2 Is Perl difficult to learn?

No, Perl is easy to start learning--and easy to keep learning. It looks
like most programming languages you're likely to have experience
with, so if you've ever written a C program, an awk script, a shell
script, or even a BASIC program, you're already partway there.

Most tasks only require a small subset of the Perl language. One of
the guiding mottos for Perl development is "there's more than one way
to do it" (TMTOWTDI, sometimes pronounced "tim toady"). Perl's
learning curve is therefore shallow (easy to learn) and long (there's
a whole lot you can do if you really want).

Finally, because Perl is frequently (but not always, and certainly not by
definition) an interpreted language, you can write your programs and test
them without an intermediate compilation step, allowing you to experiment
and test/debug quickly and easily. This ease of experimentation flattens
the learning curve even more.

Things that make Perl easier to learn: Unix experience, almost any kind
of programming experience, an understanding of regular expressions, and
the ability to understand other people's code. If there's something you
need to do, then it's probably already been done, and a working example is
usually available for free. Don't forget Perl modules, either.
They're discussed in Part 3 of this FAQ, along with CPAN, which is
discussed in Part 2.

=head2 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?

Favorably in some areas, unfavorably in others. Precisely which areas
are good and bad is often a personal choice, so asking this question
on Usenet runs a strong risk of starting an unproductive Holy War.

Probably the best thing to do is try to write equivalent code to do a
set of tasks. These languages have their own newsgroups in which you
can learn about (but hopefully not argue about) them.

Some comparison documents can be found at http://www.perl.com/doc/FMTEYEWTK/versus/
if you really can't stop yourself.

=head2 Can I do [task] in Perl?

Perl is flexible and extensible enough for you to use on virtually any
task, from one-line file-processing tasks to large, elaborate systems.
For many people, Perl serves as a great replacement for shell scripting.
For others, it serves as a convenient, high-level replacement for most of
what they'd program in low-level languages like C or C++. It's ultimately
up to you (and possibly your management) which tasks you'll use Perl
for and which you won't.

If you have a library that provides an API, you can make any component
of it available as just another Perl function or variable using a Perl
extension written in C or C++ and dynamically linked into your main
perl interpreter. You can also go the other direction, and write your
main program in C or C++, and then link in some Perl code on the fly,
to create a powerful application. See L<perlembed>.

That said, there will always be small, focused, special-purpose
languages dedicated to a specific problem domain that are simply more
convenient for certain kinds of problems. Perl tries to be all things
to all people, but nothing special to anyone. Examples of specialized
languages that come to mind include prolog and matlab.

=head2 When shouldn't I program in Perl?

When your manager forbids it--but do consider replacing them :-).

Actually, one good reason is when you already have an existing
application written in another language that's all done (and done
well), or you have an application language specifically designed for a
certain task (e.g. prolog, make).

For various reasons, Perl is probably not well-suited for real-time
embedded systems, low-level operating systems development work like
device drivers or context-switching code, complex multi-threaded
shared-memory applications, or extremely large applications. You'll
notice that perl is not itself written in Perl.

Perl remains fundamentally a dynamically typed language, not
a statically typed one. You certainly won't be chastised if you don't
trust nuclear-plant or brain-surgery monitoring code to it. And Larry
will sleep easier, too--Wall Street programs not withstanding. :-)

=head2 What's the difference between "perl" and "Perl"?

One bit. Oh, you weren't talking ASCII? :-) Larry now uses "Perl" to
signify the language proper and "perl" the implementation of it, i.e.
the current interpreter. Hence Tom's quip that "Nothing but perl can
parse Perl."

Before the first edition of I<Programming perl>, people commonly
referred to the language as "perl", and its name appeared that way in
the title because it referred to the interpreter. In the book, Randal
Schwartz capitalised the language's name to make it stand out better
when typeset. This convention was adopted by the community, and the
second edition became I<Programming Perl>, using the capitalized
version of the name to refer to the language.

You may or may not choose to follow this usage. For example,
parallelism means "awk and perl" and "Python and Perl" look good, while
"awk and Perl" and "Python and perl" do not. But never write "PERL",
because perl is not an acronym, apocryphal folklore and post-facto
expansions notwithstanding.

=head2 Is it a Perl program or a Perl script?

Larry doesn't really care. He says (half in jest) that "a script is
what you give the actors. A program is what you give the audience."

Originally, a script was a canned sequence of normally interactive
commands--that is, a chat script. Something like a UUCP or PPP chat
script or an expect script fits the bill nicely, as do configuration
scripts run by a program at its start up, such F<.cshrc> or F<.ircrc>,
for example. Chat scripts were just drivers for existing programs,
not stand-alone programs in their own right.

A computer scientist will correctly explain that all programs are
interpreted and that the only question is at what level. But if you
ask this question of someone who isn't a computer scientist, they might
tell you that a I<program> has been compiled to physical machine code
once and can then be run multiple times, whereas a I<script> must be
translated by a program each time it's used.

Now that "script" and "scripting" are terms that have been seized by
unscrupulous or unknowing marketeers for their own nefarious purposes,
they have begun to take on strange and often pejorative meanings,
like "non serious" or "not real programming". Consequently, some Perl
programmers prefer to avoid them altogether.

=head2 What is a JAPH?

(contributed by brian d foy)

JAPH stands for "Just another Perl hacker,", which Randal Schwartz used
to sign email and usenet messages starting in the late 1980s. He
previously used the phrase with many subjects ("Just another x hacker,"),
so to distinguish his JAPH, he started to write them as Perl programs:

        print "Just another Perl hacker,";

Other people picked up on this and started to write clever or obfuscated
programs to produce the same output, spinning things quickly out of
control while still providing hours of amusement for their creators and
readers.

CPAN has several JAPH programs at http://www.cpan.org/misc/japh .

=head2 Where can I get a list of Larry Wall witticisms?

(contributed by brian d foy)

Google "larry wall quotes"! You might even try the "I feel lucky" button.
:)

Wikiquote has the witticisms from Larry along with their source,
including his usenet postings and source code comments.

If you want a plain text file, try
http://www.cpan.org/misc/lwall-quotes.txt.gz .

=head2 How can I convince others to use Perl?

(contributed by brian d foy)

Appeal to their self interest! If Perl is new (and thus scary) to them,
find something that Perl can do to solve one of their problems. That
might mean that Perl either saves them something (time, headaches, money)
or gives them something (flexibility, power, testability).

In general, the benefit of a language is closely related to the skill of
the people using that language. If you or your team can be more faster,
better, and stronger through Perl, you'll deliver more value. Remember,
people often respond better to what they get out of it. If you run
into resistance, figure out what those people get out of the other
choice and how Perl might satisfy that requirement.

You don't have to worry about finding or paying for Perl; it's freely
available and several popular operating systems come with Perl. Community
support in places such as Perlmonks ( http://www.perlmonks.com )
and the various Perl mailing lists ( http://lists.perl.org ) means that
you can usually get quick answers to your problems.

Finally, keep in mind that Perl might not be the right tool for every
job. You're a much better advocate if your claims are reasonable and
grounded in reality. Dogmatically advocating anything tends to make
people discount your message. Be honest about possible disadvantages
to your choice of Perl since any choice has trade-offs.

You might find these links useful:

=over 4

=item * http://perltraining.com.au/whyperl.html

=item * http://www.perl.org/advocacy/whyperl.html

=back

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.

This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.

Irrespective of its distribution, all code examples here are in the public
domain. You are permitted and encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you
see fit. A simple comment in the code giving credit to the FAQ would
be courteous but is not required.