Faster permutation algorithms

[perl5.git] / pod / perlfaq1.pod

=head1 NAME

perlfaq1 - General Questions About Perl ($Revision: 1.23 $, $Date: 1999/05/23 16:08:30 $)

=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
nntp://news.perl.com/perl.porters-gw/ and the Deja archive at
http://www.deja.com/ using the perl.porters-gw newsgroup, or you can
subscribe to the mailing list by sending perl5-porters-request@perl.org
a subscription request.

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?

You should definitely use version 5.  Version 4 is old, limited, and
no longer maintained; its last patch (4.036) was in 1992, long ago and
far away.  Sure, it's stable, but so is anything that's dead; in fact,
perl4 had been called a dead, flea-bitten camel carcass.  The most recent
production release is 5.6 (although 5.005_03 is still supported).
The most cutting-edge development release is 5.7.  Further references
to the Perl language in this document refer to the production release
unless otherwise specified.  There may be one or more official bug fixes
by the time you read this, and also perhaps some experimental versions
on the way to the next release.  All releases prior to 5.004 were subject
to buffer overruns, a grave security issue.

=head2 What are perl4 and perl5?

Perl4 and perl5 are informal names for different versions of the Perl
programming language.  It's easier to say "perl5" than it is to say
"the 5(.004) release of Perl", but some people have interpreted this
to mean there's a language called "perl5", which isn't the case.
Perl5 is merely the popular name for the fifth major release (October 1994),
while perl4 was the fourth major release (March 1991).  There was also a
perl1 (in January 1988), a perl2 (June 1988), and a perl3 (October 1989).

The 5.0 release is, essentially, a ground-up rewrite of the original
perl source code from releases 1 through 4.  It has been modularized,
object-oriented, tweaked, trimmed, and optimized until it almost doesn't
look like the old code.  However, the interface is mostly the same, and
compatibility with previous releases is very high. 
See L<perltrap/"Perl4 to Perl5 Traps">.

To avoid the "what language is perl5?" confusion, some people prefer to
simply use "perl" to refer to the latest version of perl and avoid using
"perl5" altogether.  It's not really that big a deal, though.

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

=head2 What is perl6?

At The Second O'Reilly Open Source Software Convention, Larry Wall            
announced Perl6 development would begin in earnest. Perl6 was an oft
used term for Chip Salzenberg's project to rewrite Perl in C++ named
Topaz. However, Topaz should not be confused with the nisus to rewrite
Perl while keeping the lessons learned from other software, as well as
Perl5, in mind.

If you have a desire to help in the crusade to make Perl a better place
then peruse the Perl6 developers page at http://www.perl.org/perl6/ and 
get involved.

The first alpha release is expected by Summer 2001.

"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 perl4 scripts run flawlessly
under perl5, 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 the new 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://language.perl.com/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.

The new, native-code compiler for Perl may eventually reduce the
limitations given in the previous statement to some degree, but understand
that 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."  You may or may not choose to follow this usage.  For
example, parallelism means "awk and perl" and "Python and Perl" look
OK, while "awk and Perl" and "Python and perl" do not.  But never
write "PERL", because perl isn't really 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.

Perl programs are (usually) neither strictly compiled nor strictly
interpreted.  They can be compiled to a byte-code form (something of a
Perl virtual machine) or to completely different languages, like C or
assembly language.  You can't tell just by looking at it whether the
source is destined for a pure interpreter, a parse-tree interpreter,
a byte-code interpreter, or a native-code compiler, so it's hard to give
a definitive answer here.

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?

These are the "just another perl hacker" signatures that some people
sign their postings with.  Randal Schwartz made these famous.  About
100 of the earlier ones are available from
http://www.perl.com/CPAN/misc/japh .

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

Over a hundred quips by Larry, from postings of his or source code,
can be found at http://www.perl.com/CPAN/misc/lwall-quotes.txt.gz .

=head2 How can I convince my sysadmin/supervisor/employees to use version 5/5.005/Perl instead of some other language?

If your manager or employees are wary of unsupported software, or
software which doesn't officially ship with your operating system, you
might try to appeal to their self-interest.  If programmers can be
more productive using and utilizing Perl constructs, functionality,
simplicity, and power, then the typical manager/supervisor/employee
may be persuaded.  Regarding using Perl in general, it's also
sometimes helpful to point out that delivery times may be reduced
using Perl compared to other languages.

If you have a project which has a bottleneck, especially in terms of
translation or testing, Perl almost certainly will provide a viable,
quick solution.  In conjunction with any persuasion effort, you
should not fail to point out that Perl is used, quite extensively, and
with extremely reliable and valuable results, at many large computer
software and hardware companies throughout the world.  In fact,
many Unix vendors now ship Perl by default.  Support is usually
just a news-posting away, if you can't find the answer in the
I<comprehensive> documentation, including this FAQ.

See http://www.perl.org/advocacy/ for more information.

If you face reluctance to upgrading from an older version of perl,
then point out that version 4 is utterly unmaintained and unsupported
by the Perl Development Team.  Another big sell for Perl5 is the large
number of modules and extensions which greatly reduce development time
for any given task.  Also mention that the difference between version
4 and version 5 of Perl is like the difference between awk and C++.
(Well, OK, maybe it's not quite that distinct, but you get the idea.)
If you want support and a reasonable guarantee that what you're
developing will continue to work in the future, then you have to run
the supported version.  As of April 2001 that probably means
running either of the releases 5.6.1 (released in April 2001) or
5.005_03 (released in March 1999), although 5.004_05 isn't that bad
if you B<absolutely> need such an old version (released in April 1999)
for stability reasons.  Anything older than 5.004_05 shouldn't be used.

Of particular note is the massive bug hunt for buffer overflow
problems that went into the 5.004 release.  All releases prior to
that, including perl4, are considered insecure and should be upgraded
as soon as possible.

In August 2000 in all Linux distributions a new security problem was
found in the optional 'suidperl' (not built or installed by default)
in all the Perl branches 5.6, 5.005, and 5.004, see
http://www.cpan.org/src/5.0/sperl-2000-08-05/
Perl maintenance releases 5.6.1 and 5.8.0 have this security hole closed.
Most, if not all, Linux distribution have patches for this
vulnerability available, see http://www.linuxsecurity.com/advisories/ ,
but the most recommendable way is to upgrade to at least Perl 5.6.1.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997, 1998, 1999, 2000, 2001 Tom Christiansen and Nathan
Torkington.  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.