326 lines
16 KiB
Plaintext
326 lines
16 KiB
Plaintext
=head1 NAME
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perlfaq1 - General Questions About Perl ($Revision: 1.20 $, $Date: 1999/01/08 04:22:09 $)
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=head1 DESCRIPTION
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This section of the FAQ answers very general, high-level questions
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about Perl.
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=head2 What is Perl?
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Perl is a high-level programming language with an eclectic heritage
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written by Larry Wall and a cast of thousands. It derives from the
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ubiquitous C programming language and to a lesser extent from sed,
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awk, the Unix shell, and at least a dozen other tools and languages.
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Perl's process, file, and text manipulation facilities make it
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particularly well-suited for tasks involving quick prototyping, system
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utilities, software tools, system management tasks, database access,
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graphical programming, networking, and world wide web programming.
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These strengths make it especially popular with system administrators
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and CGI script authors, but mathematicians, geneticists, journalists,
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and even managers also use Perl. Maybe you should, too.
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=head2 Who supports Perl? Who develops it? Why is it free?
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The original culture of the pre-populist Internet and the deeply-held
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beliefs of Perl's author, Larry Wall, gave rise to the free and open
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distribution policy of perl. Perl is supported by its users. The
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core, the standard Perl library, the optional modules, and the
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documentation you're reading now were all written by volunteers. See
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the personal note at the end of the README file in the perl source
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distribution for more details. See L<perlhist> (new as of 5.005)
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for Perl's milestone releases.
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In particular, the core development team (known as the Perl Porters)
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are a rag-tag band of highly altruistic individuals committed
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to producing better software for free than you could hope to
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purchase for money. You may snoop on pending developments via
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nntp://news.perl.com/perl.porters-gw/ and the Deja News archive at
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http://www.dejanews.com/ using the perl.porters-gw newsgroup, or you can
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subscribe to the mailing list by sending perl5-porters-request@perl.org
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a subscription request.
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While the GNU project includes Perl in its distributions, there's no
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such thing as "GNU Perl". Perl is not produced nor maintained by the
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Free Software Foundation. Perl's licensing terms are also more open
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than GNU software's tend to be.
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You can get commercial support of Perl if you wish, although for most
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users the informal support will more than suffice. See the answer to
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"Where can I buy a commercial version of perl?" for more information.
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=head2 Which version of Perl should I use?
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You should definitely use version 5. Version 4 is old, limited, and
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no longer maintained; its last patch (4.036) was in 1992, long ago and
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far away. Sure, it's stable, but so is anything that's dead; in fact,
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perl4 had been called a dead, flea-bitten camel carcass. The most recent
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production release is 5.005_02 (although 5.004_04 is still supported).
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The most cutting-edge development release is 5.005_54. Further references
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to the Perl language in this document refer to the production release
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unless otherwise specified. There may be one or more official bug
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fixes for 5.005_02 by the time you read this, and also perhaps some
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experimental versions on the way to the next release. All releases
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prior to 5.004 were subject to buffer overruns, a grave security issue.
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=head2 What are perl4 and perl5?
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Perl4 and perl5 are informal names for different versions of the Perl
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programming language. It's easier to say "perl5" than it is to say
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"the 5(.004) release of Perl", but some people have interpreted this
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to mean there's a language called "perl5", which isn't the case.
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Perl5 is merely the popular name for the fifth major release (October 1994),
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while perl4 was the fourth major release (March 1991). There was also a
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perl1 (in January 1988), a perl2 (June 1988), and a perl3 (October 1989).
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The 5.0 release is, essentially, a ground-up rewrite of the original
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perl source code from releases 1 through 4. It has been modularized,
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object-oriented, tweaked, trimmed, and optimized until it almost doesn't
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look like the old code. However, the interface is mostly the same, and
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compatibility with previous releases is very high. See L<perltrap/"Perl4
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to Perl5 Traps">.
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To avoid the "what language is perl5?" confusion, some people prefer to
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simply use "perl" to refer to the latest version of perl and avoid using
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"perl5" altogether. It's not really that big a deal, though.
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See L<perlhist> for a history of Perl revisions.
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=head2 What is perl6?
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Perl6 is a semi-jocular reference to the Topaz project. Headed by Chip
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Salzenberg, Topaz is yet-another ground-up rewrite of the current release
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of Perl, one whose major goal is to create a more maintainable core than
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found in release 5. Written in nominally portable C++, Topaz hopes to
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maintain 100% source-compatibility with previous releases of Perl but to
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run significantly faster and smaller. The Topaz team hopes to provide
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an XS compatibility interface to allow most XS modules to work unchanged,
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albeit perhaps without the efficiency that the new interface uowld allow.
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New features in Topaz are as yet undetermined, and will be addressed
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once compatibility and performance goals are met.
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If you are a hard-working C++ wizard with a firm command of Perl's
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internals, and you would like to work on the project, send a request to
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perl6-porters-request@perl.org to subscribe to the Topaz mailing list.
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There is no ETA for Topaz. It is expected to be several years before it
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achieves enough robustness, compatibility, portability, and performance
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to replace perl5 for ordinary use by mere mortals.
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=head2 How stable is Perl?
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Production releases, which incorporate bug fixes and new functionality,
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are widely tested before release. Since the 5.000 release, we have
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averaged only about one production release per year.
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Larry and the Perl development team occasionally make changes to the
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internal core of the language, but all possible efforts are made toward
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backward compatibility. While not quite all perl4 scripts run flawlessly
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under perl5, an update to perl should nearly never invalidate a program
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written for an earlier version of perl (barring accidental bug fixes
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and the rare new keyword).
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=head2 Is Perl difficult to learn?
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No, Perl is easy to start learning -- and easy to keep learning. It looks
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like most programming languages you're likely to have experience
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with, so if you've ever written an C program, an awk script, a shell
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script, or even BASIC program, you're already part way there.
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Most tasks only require a small subset of the Perl language. One of
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the guiding mottos for Perl development is "there's more than one way
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to do it" (TMTOWTDI, sometimes pronounced "tim toady"). Perl's
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learning curve is therefore shallow (easy to learn) and long (there's
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a whole lot you can do if you really want).
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Finally, because Perl is frequently (but not always, and certainly not by
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definition) an interpreted language, you can write your programs and test
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them without an intermediate compilation step, allowing you to experiment
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and test/debug quickly and easily. This ease of experimentation flattens
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the learning curve even more.
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Things that make Perl easier to learn: Unix experience, almost any kind
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of programming experience, an understanding of regular expressions, and
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the ability to understand other people's code. If there's something you
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need to do, then it's probably already been done, and a working example is
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usually available for free. Don't forget the new perl modules, either.
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They're discussed in Part 3 of this FAQ, along with CPAN, which is
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discussed in Part 2.
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=head2 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
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Favorably in some areas, unfavorably in others. Precisely which areas
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are good and bad is often a personal choice, so asking this question
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on Usenet runs a strong risk of starting an unproductive Holy War.
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Probably the best thing to do is try to write equivalent code to do a
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set of tasks. These languages have their own newsgroups in which you
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can learn about (but hopefully not argue about) them.
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Some comparison documents can be found at http://language.perl.com/versus/
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if you really can't stop yourself.
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=head2 Can I do [task] in Perl?
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Perl is flexible and extensible enough for you to use on virtually any
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task, from one-line file-processing tasks to large, elaborate systems.
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For many people, Perl serves as a great replacement for shell scripting.
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For others, it serves as a convenient, high-level replacement for most of
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what they'd program in low-level languages like C or C++. It's ultimately
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up to you (and possibly your management) which tasks you'll use Perl
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for and which you won't.
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If you have a library that provides an API, you can make any component
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of it available as just another Perl function or variable using a Perl
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extension written in C or C++ and dynamically linked into your main
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perl interpreter. You can also go the other direction, and write your
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main program in C or C++, and then link in some Perl code on the fly,
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to create a powerful application. See L<perlembed>.
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That said, there will always be small, focused, special-purpose
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languages dedicated to a specific problem domain that are simply more
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convenient for certain kinds of problems. Perl tries to be all things
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to all people, but nothing special to anyone. Examples of specialized
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languages that come to mind include prolog and matlab.
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=head2 When shouldn't I program in Perl?
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When your manager forbids it -- but do consider replacing them :-).
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Actually, one good reason is when you already have an existing
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application written in another language that's all done (and done
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well), or you have an application language specifically designed for a
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certain task (e.g. prolog, make).
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For various reasons, Perl is probably not well-suited for real-time
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embedded systems, low-level operating systems development work like
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device drivers or context-switching code, complex multi-threaded
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shared-memory applications, or extremely large applications. You'll
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notice that perl is not itself written in Perl.
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The new, native-code compiler for Perl may eventually reduce the
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limitations given in the previous statement to some degree, but understand
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that Perl remains fundamentally a dynamically typed language, not
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a statically typed one. You certainly won't be chastised if you don't
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trust nuclear-plant or brain-surgery monitoring code to it. And Larry
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will sleep easier, too -- Wall Street programs not withstanding. :-)
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=head2 What's the difference between "perl" and "Perl"?
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One bit. Oh, you weren't talking ASCII? :-) Larry now uses "Perl" to
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signify the language proper and "perl" the implementation of it,
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i.e. the current interpreter. Hence Tom's quip that "Nothing but perl
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can parse Perl." You may or may not choose to follow this usage. For
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example, parallelism means "awk and perl" and "Python and Perl" look
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ok, while "awk and Perl" and "Python and perl" do not. But never
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write "PERL", because perl isn't really an acronym, aprocryphal
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folklore and post-facto expansions notwithstanding.
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=head2 Is it a Perl program or a Perl script?
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Larry doesn't really care. He says (half in jest) that "a script is
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what you give the actors. A program is what you give the audience."
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Originally, a script was a canned sequence of normally interactive
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commands, that is, a chat script. Something like a uucp or ppp chat
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script or an expect script fits the bill nicely, as do configuration
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scripts run by a program at its start up, such F<.cshrc> or F<.ircrc>,
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for example. Chat scripts were just drivers for existing programs,
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not stand-alone programs in their own right.
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A computer scientist will correctly explain that all programs are
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interpreted, and that the only question is at what level. But if you
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ask this question of someone who isn't a computer scientist, they might
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tell you that a I<program> has been compiled to physical machine code
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once, and can then be run multiple times, whereas a I<script> must be
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translated by a program each time it's used.
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Perl programs are (usually) neither strictly compiled nor strictly
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interpreted. They can be compiled to a byte-code form (something of a
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Perl virtual machine) or to completely different languages, like C or
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assembly language. You can't tell just by looking at it whether the
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source is destined for a pure interpreter, a parse-tree interpreter,
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a byte-code interpreter, or a native-code compiler, so it's hard to give
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a definitive answer here.
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Now that "script" and "scripting" are terms that have been seized by
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unscrupulous or unknowing marketeers for their own nefarious purposes,
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they have begun to take on strange and often pejorative meanings,
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like "non serious" or "not real programming". Consequently, some perl
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programmers prefer to avoid them altogether.
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=head2 What is a JAPH?
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These are the "just another perl hacker" signatures that some people
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sign their postings with. Randal Schwartz made these famous. About
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100 of the earlier ones are available from
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http://www.perl.com/CPAN/misc/japh .
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=head2 Where can I get a list of Larry Wall witticisms?
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Over a hundred quips by Larry, from postings of his or source code,
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can be found at http://www.perl.com/CPAN/misc/lwall-quotes.txt.gz .
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Newer examples can be found by perusing Larry's postings:
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http://x1.dejanews.com/dnquery.xp?QRY=*&DBS=2&ST=PS&defaultOp=AND&LNG=ALL&format=terse&showsort=date&maxhits=100&subjects=&groups=&authors=larry@*wall.org&fromdate=&todate=
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=head2 How can I convince my sysadmin/supervisor/employees to use version (5/5.005/Perl instead of some other language)?
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If your manager or employees are wary of unsupported software, or
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software which doesn't officially ship with your Operating System, you
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might try to appeal to their self-interest. If programmers can be
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more productive using and utilizing Perl constructs, functionality,
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simplicity, and power, then the typical manager/supervisor/employee
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may be persuaded. Regarding using Perl in general, it's also
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sometimes helpful to point out that delivery times may be reduced
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using Perl, as compared to other languages.
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If you have a project which has a bottleneck, especially in terms of
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translation or testing, Perl almost certainly will provide a viable,
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and quick solution. In conjunction with any persuasion effort, you
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should not fail to point out that Perl is used, quite extensively, and
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with extremely reliable and valuable results, at many large computer
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software and/or hardware companies throughout the world. In fact,
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many Unix vendors now ship Perl by default, and support is usually
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just a news-posting away, if you can't find the answer in the
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I<comprehensive> documentation, including this FAQ.
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See http://www.perl.org/advocacy/ for more information.
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If you face reluctance to upgrading from an older version of perl,
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then point out that version 4 is utterly unmaintained and unsupported
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by the Perl Development Team. Another big sell for Perl5 is the large
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number of modules and extensions which greatly reduce development time
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for any given task. Also mention that the difference between version
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4 and version 5 of Perl is like the difference between awk and C++.
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(Well, ok, maybe not quite that distinct, but you get the idea.) If you
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want support and a reasonable guarantee that what you're developing
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will continue to work in the future, then you have to run the supported
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version. That probably means running the 5.005 release, although 5.004
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isn't that bad. Several important bugs were fixed from the 5.000 through
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5.003 versions, though, so try upgrading past them if possible.
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Of particular note is the massive bughunt for buffer overflow
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problems that went into the 5.004 release. All releases prior to
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that, including perl4, are considered insecure and should be upgraded
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as soon as possible.
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=head1 AUTHOR AND COPYRIGHT
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Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
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All rights reserved.
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When included as an integrated part of the Standard Distribution
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of Perl or of its documentation (printed or otherwise), this work is
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covered under Perl's Artistic Licence. For separate distributions of
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all or part of this FAQ outside of that, see L<perlfaq>.
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Irrespective of its distribution, all code examples here are public
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domain. You are permitted and encouraged to use this code and any
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derivatives thereof in your own programs for fun or for profit as you
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see fit. A simple comment in the code giving credit to the FAQ would
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be courteous but is not required.
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