Wiki: PerlDesignPatterns

etc. but does not include extensive style markup. The DocBook source file is processed via tools to produce XHTML, HTML, and through and intermediate form to PDF. This means that one source file can produce both HTML for the Web and also PDF for documents that are printable. If your company produces and/or uses many documents you should consider this technology.

CvsQuickRef

file may be "." to mean the current directory and everything below it.

  # all the time
  cvs commit file          sends changes to repository
  cvs update file          freshens your files
  
  # useful too
  cvs diff files           compare your files to repository
  cvs log file             comments on changes in each file
  cvs update file          merge in changes from the repository
  cvs update -j currentrev -j previousrev file 
                           regress to a previous version
  
  # not very often
  cvs tag string files
  cvs add files            introduce to repository pending commit
  cvs remove files         nuke file from repository
  cvs checkout file        get a file or project initially

ClassNaming

APPENDIX C

PlanToThrowOneAway

Concept from TheMythicalManMonth.

//I gave desperate warnings against the obscurity, the complexity, and over-ambition of the new design, but my warnings went unheeded. I conclude that there are two ways of constructing a software design: One way is to make it so simple there are obviously no deficiencies and the other way is to make it so complicated that there are no obvious deficiencies// Tony Hoare, Turing Aware Lecture on Ada, 1980

ArtAndScience

Programs written following strict formal method and careful design seldom if ever make works of art: TopDownDesign.

An attempt to do so ignores important NecessaryComplexity.

Science accels at problems requiring strict understanding of the factors involved and

TheCodeIsThePrototype: Suggesting that an entire project be built in order to learn how to build sounds absurd and will get you laughed out of a meeting. This doesn't sound so absurd considering most projects are considered failures and aborted.

PlanningIsNpComplete: Art accels at problems without a single deturministic solution. Complex comprimise on poorly understood forces is the hullmark of a creative solution. "Poorly understood" isn't a bad thing. Humans know all kinds of things that they don't know how they know. Almost anyone can tie their shoes, walk, and ride a bike, but we lack intricate knowledge of how we learned these things. We don't know enough about the processes to program computers to do them. It doesn't mean we don't know them, only that they haven't been formalized. Tapping into non-formal knowlege is critical for art.

Formal knowledge, when applied opressively, hinders art. When used as a starting point, it can meld with non-formal knowledge. Sometimes a hair of the camel that bit you is the best remedy. Concepts of shadow, perspective, movement can be approximated with good affect in sketch. Knowledge about light, geometetry and perception can be applied creatively to improve ideas. In the same way, hashing algorithms, cryptography and pattern recognition can be used as starting points for creative program ideas: knowledge bots, secture distributed networks, and computer game opponents. Knowledge of architectural structures can be applied creatively by programmers seeking to build programs that are required to be larger and more agile than formal systems could mechanically churn out.

I hate this page XXX. I'm totally screwing it up.

It was once thought that the imaginitive outlook of the artist was death for the scientist. And the logic of science seemed to spell doom to all possible artistic flights of fancy. -- Robert E. Meuller, The Science of Art

Going into an introductory drawing class, you'll learn about negitive space, implied lines, gestural drawing, shadow and weight, blending color, and all sorts of goodies. The observations that lead to formilization of these techniques are scientific. These methods, when applied, always have the prescribed effects, unless a greater force is acting in another direction. Since so much is understood about light and visual perception, one might say that "art" has been turned into a "science".

There ideas are merely tools, means to an end. The end is completely undefined, and the tools can always be combined in new ways. Neglecting to use a tool that would seem obvious can lead to powerful self-discovery, unique results, and entire revolutions.

As it turns out, not only does creativity have everything to do with creating art, it has everything to do with creating science as well. Science merely helps us control the medium by understanding it better. ZenAndTheArtOfMotorcycleMaintenance has a recurring theme the formulation of hypothesis is creative.

This needs better explaination. XXX.

How do they develop their skill? The best filmmakers through the years usually seem to have learned their art in comparatively primitive circumstances, often in countries with a limited movie industry. And in recent years the most important things we have learned about programming seem to have originated with people who did not have access to very large computers. The moral of the story is ... we should make use of the idea of limited resources in our education. -- Knuth, LiterateProgramming

Ironic. Sculpture is much more difficult than drawing with charcoal. Having control over 3 dimentional space, and only needing to create the shape to have lighting play itself out should make the creative process easier - but it doesn't. Many old timers complain that video games in the arcades and for old consoles like the Atari 2600 showed constant innovation, and were able to go in a new, unique directions, untethered to the limitations of reality. RobertPirsig, in ZenAndTheArtOfMotorcycleMaintenance, writes about a student who is stymied to draw a downtown street from her childhood, but it pours out of her when she is constrained to drawing a single brick on a specific building.

Many programmers, given GUI building tools, an integrated development environment, and a massive library of reusable components first make a simple program that shows off the power of the language or computer is some gaudy way, then quickly loose interest. Creativity is about solving problems, and actively redefining your limits. It is pointless if it isn't firmly rooted in reality - even if it is tangent to reality, or askew of reality, it is still defined by reality. Having no connection with Earthly matters robs one of all inspiration. Writing a simple program to do a small but useful task is engaging. Trying to do significant amounts of work with artificially contrained space - very little memory, time, or lines of code - drawns out illusions of having all of the resources of a dedicated computer that has no other useful task and connects us with the humanity of the urgency of deadlines, the millions of lines of code a programmer will have to type out in their life, the inability to get a new computer approved even though your 7 year old system is costing the company tens of thousands of dollars worth of lost productivity each year. Pushing the limits of a computer pushes our limits as well. It is often said that a modern day programmer would be completely helpless on an old system, such as a mini with 8192 words of core memory, a system that was once the pride and joy of the Research & Development department at many a major company. We aren't any better programmers for being spoilt on bloated tools and formal methods. When programming a small system, there are always a few tricks that help save instructions, but no method can touch the power of creative problem solving.

A person writing a Design Patterns book might glorify themselves and think of themselves as a sort of technology priest. Preachers will promise you things to come, and condem your sins. I hope this book to be inspirational by preaching of heavenly tricks and warning of perilous traps.

HowPerlDiffersFromC

Perl is a lot like C, which makes C programmers wonder where the heck strlen() got off to. Some of the most noticable things that must be overcome before perl will accept your psuedo-C are:

Dollar signs in front of the scalars.

Yeah, I know. Its a BASIC thing that was perpetuated by sh. In Perl, they are called sigils. It does serve a purpose: by allowing you to create functions, I/O handles, arrays, hashes and scalars (which might contain references), you can write more obscure perl. It also allows you to write expressions using bare words:

  $a = wussup;

Bare words are unqouted things that appear in the text that aren't function names. They get treated literally for lack of anything else to do with them. That makes that expression the same as:

  $a = "wussup";

This is considered bad style in all but one case (hash table indices), but it also serves to prevent variables from not having to have funny symbols in front of them.

Since variables can be introduced at any time, including leaf nodes on complex data structures built out of references, the sigils tell perl what we're working with. @foo = function(); tells perl that you want a list of things, while $foo = function(); asks for only one. The situation is similar when fetching the value from variables.

strlen() and sizeof()

length() serves both strings and arrays.

Command Line Arguments

There is no specific function which is called when a Perl program is run. Instead, Perl runs the code from top to bottom, executing whatever it finds, including subroutine definitions. Therefore there is no function to pass arguments to. @ARGV has been selected for this purpose. Use $ARGV[0] to get at the first element, $ARGV[1] to get at the second, and so on. Note that the sigil in front of the variable changes depending on what you want back from it, not how it is actually declared. More specifically, the first element of @ARGV is a scalar, and that is what we're fetching.

Comments

Comments begin with the hash symbol (or number sign, or pound sign, for you brits) and end at the end of the line. Block quotes are attainable using the POD syntax. See PerlDoc for more information. Comments quoted this way will become part of the documentation of the module or program, much like how javadoc processes comments into documentation. Thus, it is advisable to use this syntax before functions, perticularly publicly accessable ones, to explain what the function does, why, and what it requires to do so.

Environment

Perl tells you about your environment in intimate detail. It is up to you to write portable code. Code that is oblivious to the system it is running on is an ideal, but is an impossible to acheive one in many cases. When workarounds are needed, you aren't restricted from performing them. See the perloc for Config, but as an example:

  # CGI.pm does this to learn about local linefeed insanity:
  $OS = $Config::Config{'osname'};

Functions without parenthsis

It is typical to see strings of expressions in Perl not unlike:

  my @files = grep { $_ !~ m/^\./ } sort readdir $fh;

This might lead one to believe that parenthesis aren't used for function arguments and curley braces some how take their place. This might be partially true. Many functions expect a list of things, and many functions return lists of things. In this example, readdir() is returning a list to sort(); sort() is sorting that and sending it through grep(); grep has a strange little block after it. This is a block of code that will be run each item in the list as a test for inclusion. sort() and map() are two other functions that accept such an code block. The block is purely optional. Since all of our lists are implicitly created and implicitly passed around, we don't ever really need to group things using parenthesis. That doesn't mean we can't:

  my @files = get_file_list($ENV{'HOME'}, '.', `pwd`);

Having optional parenthesis does create a certain amount of confusion. People are often tempted to write expressions like:

  print (32/8)+10; # this prints "4"

The space after print() means nothing. It still attaches itself to the single argument, 32/8. What probably meant is:

  print((32/8)+10); # this prints "14"

The && operator also causes problems when parenthesis aren't used on lists of numbers. The && operating will work on the last thing in the list, rather than the return value of the function:

  sprintf "The number is: %d\n", 0 && return; # this won't return

The obvious thing to do is introduce parenthesis, but you don't have to - Perl provides the and and or operators, which have very low precidence:

  sprintf "The number is: %d\n", 0 and return; # this will return

Readability can suffer, but not having to match up parenthesis all of the time saves a lot of bouncing around in the code. In case of doubt, use the parenthesis.

$foo[$i] doesn't subscript strings

$foo[$i] would return the $ith element of @foo. Perl doesn't consider $foo to be any sort of an array at all. When hand converting C code to Perl, this is a real pain, because it requires so much typing and occurs so often. The solution is:

  substr($foo, $i, 1);

This well Perl that we want a string of 1 element start at $ith element of $foo. Since strings aren't arrays of characters, this returns a string of one character. Use ord() and chr() to convert between ASCII values and very small strings.

Global, local, and lexical variables

In C, all variables are either global, visible to the source code file but otherwise global, visible to only the function they were defined static in, or else allocated on the stack as an "automatic variable".

Global: Variables not declared but just used also have the scope of the entire file. So do variables declared local at the top of the file (unless the package is changed, which is a special case).
Private to the file: variables declared my at the top of the file are visible only in that file. This is similar to declaring a variable static outside of a function in C.
Static variables: This isn't officially supported as an atomic feature, but is fairly easy to do in a couple of steps. See LexicalsMakeSense.
Automatic variables: my variables declared inside of a function serve the same purpose, but many others. They do a neat trick which combines the automatic allocation of automatic variables with the persistancy of static variables. See LexicalsMakeSense.

Module library

At compile time, a local repository for modules is established. This is like /usr/include, except it contains both code and definitions. Perl defaults to putting it in /usr/local/lib/perl5, but Linux distros like to move it up to /usr/lib/perl5. Poking through there and running perldoc on each item yields thrilling insights into Perl. Much of what is apparently implemented in core just modules that are used on demand. For instance, the pragmas like use strict are implemented mostly in terms of small modules. Downloading and installing modules from CPAN expands this collection, though in a subdirectory: /usr/local/lib/perl5/site_perl. You may also wish to add your own homebrew modules under there.

No Preprocessor

Actually, there is an interface to cpp available as a source filter, but its something you have to go and install. XXX the location of it is...?

A common trick when writing modules that are used frequently is deffering compilation of parts of code until they are first used. See AutoSplit for more information. Constants can be established using this little trick:

  my $pi = \3.14159; # I'm not a math geek

You can also read in Perl code that defines variables using the require statement.

Primitive Data Types

As mentioned, there is no character datatype. Scalars, Arrays, Hashes, Globs, File handles, Directory handles and References (a type of scalar value) are the basic datatypes. Complex datatypes can be built from these (unlike in Perl 4). References can include object references, regular expression references, code references, references to other primitive datatypes, and many other things.

Reference types are pass-by-value. No copies are made of the objects they refer to when the reference is copy or passed to a subroutine. However, Perl tracks references, and the thing being refered to won't go away while anyone can see it. Perl objects are polite and only sneak off when no one is looking unless asked to garrote themselves.

Scalars are floating point values, integers, strings, or refrences as needed. During the life of a scalar, it will typically bounce back and forth between being a string and a numeric value. In fact, Perl caches the numeric representation of strings containg numbers. For speed, Perl can be told not to use floating point representations: say use int; at the top of your file.

Functions take exactly one kind of argument: an array. Each member of the array can have its own value. In fact, a function may receive an array of exactly one thing: an array reference. Perl creates arrays on the fly like so:

  @array = (1, 2, 3, 'hi');

However, arrays that aren't passed by reference get copied. That means:

  function(9, 10, @array);

...//function()// shouldn't expect to get a reference to @array. Instead, everything inside of array will be copied onto the end of function()'s argument list, making it equivilent to saying:

  function(9, 10, 1, 2, 3, 'hi');

Since so many of Perl's built in functions work on plain old flat lists, and it is easy to compose lists from many sources to feed to a function, this is officially considered a feature.

Checking Objects For Equality

Good luck. If you have a reference, you need intimate knowledge of it to compare it value-for-value to a like reference. When writing your own objects, using an EqualsInterface is highly recommended.

Perl has no Pointers... Well...

Perl does automatic array expansion, autovivication (creation on demand) of elements in datastructures and variables, and lots of other fun DynamicLanguage stuff. If you really want to blow a smoking hole through your footsies, there is a PeekPoke module... Perl grows arrays as needed. Attempting to access a negitive array element addresses the array relative the end.

undef

The default value for variables is null. With warnings on, enabled by use warnings or passing perl the -w switch, attempts to read values from variables that have not yet had a value assigned results in a warning. You yourself can mark a variable as having no value:

  use warnings;
  my $sum = 0;
  my @prices = (10.95, 15.95, 5.95, 25.95, 45.95);
  foreach my $price (@prices) {
    $sum += $price;
  }
  print "$sum was yesterdays price. It isn't valid anymore. Sorry! Offer expired!\n";
  $sum = undef;
  print "The total for this order is: $sum\n"; # this gives an error message

The text of the error was "Use of uninitialized value in concatenation (.) or string at - line 9.".

  if(undef == 0) { print "True\n"; }

  my $a = undef;
  if($a == 0) { print "True\n"; }

Logically, undef == 0. Many built in functions return a number on success and undef on failure: 0 is a possible success return value. So is the null string, ''. Rather than testing for truth, you must test for definidness:

  if(defined my $line = <>) {
    # deal with more frivelous user demands
  }
  # user has shut up now and hit Control-D or closed the connection

defined() returns true if its argument is defined and false if its argument is in fact not.

References

Perl implements reference counting. Reference counting is fast, saves the bother of manually having to allocate and free memory (and other resources if object destructors are used). Garbage collection correctly copes with the situation where two structures reference each other. With reference counts, each structure has a reference on it, so it is never freed, even when the two structure aren't visible from anywhere else:

  do {
    my $a = [1, 2, 3];
    my $b = [4, 5, 6];
    $a->[4] = $b;
    $b->[4] = $a; # uh oh!
  }
  # $a and $b still live on, even though they are out of scope

You might not notice if you accidently do a few of these babies in code that runs only briefly then exits, like a CGI script. This can leak memory (allocate memory never to be freed) and ultimately expend all of the available resources on a long-running application, like a daemon. See DaemonProcess.

Arrays

Arrays are minipulated by value or by reference:

  int stuff[] = {1, 2, 3, 4};    /* an array in C, by reference */
  int $stuff = [1, 2, 3, 4];     # an array in Perl, by reference 
  my @stuff = (1, 2, 3, 4);      # an array in Perl, by value

If minipulated by reference, only information about where to find the array is passed around, and only one copy of the actual array exists that is shared by everyone who knows where to find it. By value, the entire array is duplicated every time it is passed anywhere, giving each function (internal or external) its own private copy to play with. One exception to that rule is the foreach() operator.

foreach() uses heavy magic to make each each element it iterates over merely an alias into the array. This is a performance optimization and a useful feature. These two code examples do the same thing:

  @ar = map { $_++} @ar;
  foreach my $i (@ar) { $i++ }

For performance, Perl programmers sometimes tell Perl to pre-grow arrays, instead of letting Perl do it on demand:

  @ar = 1000; # allocate 1000 slots

Arrays can be multidimentional. C arrays allocated thus:

  char message[10][255];    /* 10 messages of 255 characters each */

...are entirely preallocated, and have no lookup table: since each message is of fixed size, C need only look for the first one at offset 0, the next one at offset 255, the next at offset 510, and so on. This is not how Perl arrays work. Arrays allocated like:

  char *message[];           /* I don't know how many messages or how long it might be */

...is a much a much closer faximile. A pointer holds the address that points to a row of characters in memory. In the same way, a Perl array can list references to other arrays. The -> operator dereferences reference types of various type. This example creates a reference (pointer) to an array, and starts populating it with references to other arrays:

  my $arrayRef = [];
  $arrayRef->[0] = [1, 2, 3];
  $arrayRef->[1] = [4, 5, 6];
  $arrayRef->[2] = [7, 8, 9];

  print $arrayRef->[1]->[1], "\n";  # prints 5, right in the middle, just like Hollywood Squares

Because it is unambigious, Perl accepts as a short hand this syntax for indicing multidimentional arrays:

  print $arrayRef->[1][1], "\n"; # like above

Note that the first -> is still required to distinguish between the reference $arrayRef and the array @arrayRef. In some languages, arrays are objects. Perl is not one of them. In other languages, arrays are never objects. Perl is not one of them either. In Perl, arrays are sometimes objects. When they are objects, they do not a provide a standard, generic API for minipulating arrays, but rather provide an API cooked up off the cuff by the creator of the object. This means that the API is more likely to be useful but less likely to be consistent.

Strings

Strings in Perl are not null-terminated arrays of characters as they are in C, nor are they neccisarily an instance of some class, though a string-reference could be. They also aren't references, but it is possible and useful to make and use string references. The methods substr(), index(), sprintf(), and length() come to mind for being useful to work on strings. There are many more XXX. Strings in Perl, like everything else in Perl not marked otherwise, are binary safe. Arbitrary binary data may be stored in them, including the null byte. utf8 strings are available sa well - use utf8 at the top of your code for that, and see perldoc utf8.

Operators

Perl supports too many operators to list in a document like this. The list of Perl operators is listed in a much longer documented, available from perldoc perlop. Many of the C operators are available. Most critically, though:

   gt lt eq ne cmp       # like >, <, ==, !=, <=>, but for strings: compares lexically or literally

   # this prints true - you probably want the "eq" operator:
   if("hello" == "world") { print "true\n"; }  

   ->  # dereference any reference type. the exact syntax depends on the reference:

   $a->[0]      # dereference an array ref
   $a->{'hi'}   # dereference a hash ref
   $a->blurgh() # dereference an object reference

It is not uncommon to mix and match them in one statement, if you know the datatypes of everything:

   $a->[0]->{'hi'}->blurgh();

If you have trouble with this, rethink what returns what reference types. This assumes that you have a scalar, $a, that is an array reference. That array holds a list of references to hashes (hash references). Those hashes each contain references to objects. Actually, that is too broad of a statement - Perl is dynamically typed, but this paragraph assumes that you've been consistent with your datastructure.

if/elsif/else statements

In Perl, the case where an else has another if after it has its own keyword. This is due to the bracketing rules: each if statement must have brackets around its body.

  if($foo == 1) {
    # do something
  } elsif($foo == 2) {
    # do something else
  } else {
    # complain and do nothing
  }

Forward references

Perl requires my variables to be declared before they are used. Nothing else is subject to this restriction. If your code calls a method, Perl holds it in good faith that that method will be defined before it is used. use and BEGIN { } disrupt the orderly pattern of parsing then running: Perl stops compiling at sight of either of those, immediately compiles and executes the module in the case of use or immediately runs the code in the BEGIN { } block, then resumes parsing where it left off.

No Structures or Unions, no Typedef

Perl does not support Structures or Unions, expecting you to construct an object, no matter how trivial, to accomplish this. Typedef does not exist: once again, use objects. Often an AbstractFactory will accomplish the same thing.

GnuFreeDocumentationLicense

GNU Free Documentation License Version 1.1, March 2000

 Copyright (C) 2000  Free Software Foundation, Inc.
     59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 Everyone is permitted to copy and distribute verbatim copies
 of this license document, but changing it is not allowed.

0. PREAMBLE

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This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

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   and required Cover Texts given in the Document's license notice.

H. Include an unaltered copy of this License. I. Preserve the section entitled "History", and its title, and add to

   it an item stating at least the title, year, new authors, and
   publisher of the Modified Version as given on the Title Page.  If
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J. Preserve the network location, if any, given in the Document for

   public access to a Transparent copy of the Document, and likewise
   the network locations given in the Document for previous versions
   it was based on.  These may be placed in the "History" section.
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K. In any section entitled "Acknowledgements" or "Dedications",

   preserve the section's title, and preserve in the section all the
   substance and tone of each of the contributor acknowledgements
   and/or dedications given therein.

L. Preserve all the Invariant Sections of the Document,

   unaltered in their text and in their titles.  Section numbers
   or the equivalent are not considered part of the section titles.

M. Delete any section entitled "Endorsements". Such a section

   may not be included in the Modified Version.

N. Do not retitle any existing section as "Endorsements"

   or to conflict in title with any Invariant Section.

If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

You may add a section entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections entitled "History" in the various original documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgements", and any sections entitled "Dedications". You must delete all sections entitled "Endorsements."

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an "aggregate", and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (c)  YEAR   YOUR NAME.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.1
      or any later version published by the Free Software Foundation;
      with the Invariant Sections being LIST THEIR TITLES, with the
      Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
      A copy of the license is included in the section entitled "GNU
      Free Documentation License".

If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being LIST"; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

CategoryBook

Click http://wiki.slowass.net/reverse.cgi?CategoryBook for an index of all pages describing a book. Any page may join this index by including the keyword 'CategoryBook'.

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Copyright (c) Scott Walters, scott@illogics.org, and others as noted. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation. A copy of the license is attached available at www.gnu.org/licenses/fdl.txt. By posting to this site, you agree to place all submitted content under the Free Documentation License.
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Perl Design Patterns

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Introduction:

Object Nuts and Bolts:

Object Inheritance Design Patterns:

Object Structure Design Patterns:

Object Framework Patterns:

Object, Lambda Hybrid Patterns:

Non-Object Patterns:

Application Features:

Application Types:

Anti-Patterns:

Refactoring:

Concepts:

Appendices:

Other Concepts and Blurbs, Or As Of Yet Unclassified:

HomePage

TinyWiki

SkipTheIntroduction

PatternLanguage

IntroductionPattern

PerlAndPatterns

PrefacePattern

PerlPatternsResources

ReferencesMeta

GuestLog

AboutPerlPatternsProject

PerlPatternsToDo

AboutQuality

AboutPerl

AboutObjects

AboutPatterns

PerlPaint

SoftwareQualityLevels

AboutFlack

PutCodeWhereItBelongs

KeepYourOptionsOpen

LivingDocumentation

TechnologyEnhancesOurLives

PlanningIsNpComplete

TestingCode

CommunicationPattern

RootCauses

AboutTheAuthor

InstanceVariables

ShortHandInstanceVariables

StaticVariables

AccessorPattern

TypeSafety

TypedVariables

PrivateFunctions

FunctionalityIsToBeShared

AbstractClass

ChainOfConstructors

TemplateMethod

NewObjectFromExisting

ModuleDistribution

ReturnYourself

StorablePattern

ReusableFunctions

InnerClasses

OverloadOperators

OverloadPattern

ObjectsAndRelationalDatabaseSystems

CompositePattern

PrototypeConcept

ExportingPattern

ImportPattern

ThreadsPattern

TestUnit

AbstractRootClasses

StateVsClass

DesignPatternCatalog

SingletonPattern

ClassAsTypeCode

DecoratorPattern

BridgePattern

ProxyPattern

AdapterPattern

FacadePattern