perlfaq1.pod

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

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, ";

Note the trailing comma and space, which allows the addition of other
JAxH clauses for his many other interests.

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 REVISION

Revision: $Revision: 10427 $

Date: $Date: 2007-12-14 00:39:01 +0100 (Fri, 14 Dec 2007) $

See L<perlfaq> for source control details and availability.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997-2007 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.