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1 =head1 NAME 2 3 perlmodstyle - Perl module style guide 4 5 =head1 INTRODUCTION 6 7 This document attempts to describe the Perl Community's "best practice" 8 for writing Perl modules. It extends the recommendations found in 9 L<perlstyle> , which should be considered required reading 10 before reading this document. 11 12 While this document is intended to be useful to all module authors, it is 13 particularly aimed at authors who wish to publish their modules on CPAN. 14 15 The focus is on elements of style which are visible to the users of a 16 module, rather than those parts which are only seen by the module's 17 developers. However, many of the guidelines presented in this document 18 can be extrapolated and applied successfully to a module's internals. 19 20 This document differs from L<perlnewmod> in that it is a style guide 21 rather than a tutorial on creating CPAN modules. It provides a 22 checklist against which modules can be compared to determine whether 23 they conform to best practice, without necessarily describing in detail 24 how to achieve this. 25 26 All the advice contained in this document has been gleaned from 27 extensive conversations with experienced CPAN authors and users. Every 28 piece of advice given here is the result of previous mistakes. This 29 information is here to help you avoid the same mistakes and the extra 30 work that would inevitably be required to fix them. 31 32 The first section of this document provides an itemized checklist; 33 subsequent sections provide a more detailed discussion of the items on 34 the list. The final section, "Common Pitfalls", describes some of the 35 most popular mistakes made by CPAN authors. 36 37 =head1 QUICK CHECKLIST 38 39 For more detail on each item in this checklist, see below. 40 41 =head2 Before you start 42 43 =over 4 44 45 =item * 46 47 Don't re-invent the wheel 48 49 =item * 50 51 Patch, extend or subclass an existing module where possible 52 53 =item * 54 55 Do one thing and do it well 56 57 =item * 58 59 Choose an appropriate name 60 61 =back 62 63 =head2 The API 64 65 =over 4 66 67 =item * 68 69 API should be understandable by the average programmer 70 71 =item * 72 73 Simple methods for simple tasks 74 75 =item * 76 77 Separate functionality from output 78 79 =item * 80 81 Consistent naming of subroutines or methods 82 83 =item * 84 85 Use named parameters (a hash or hashref) when there are more than two 86 parameters 87 88 =back 89 90 =head2 Stability 91 92 =over 4 93 94 =item * 95 96 Ensure your module works under C<use strict> and C<-w> 97 98 =item * 99 100 Stable modules should maintain backwards compatibility 101 102 =back 103 104 =head2 Documentation 105 106 =over 4 107 108 =item * 109 110 Write documentation in POD 111 112 =item * 113 114 Document purpose, scope and target applications 115 116 =item * 117 118 Document each publically accessible method or subroutine, including params and return values 119 120 =item * 121 122 Give examples of use in your documentation 123 124 =item * 125 126 Provide a README file and perhaps also release notes, changelog, etc 127 128 =item * 129 130 Provide links to further information (URL, email) 131 132 =back 133 134 =head2 Release considerations 135 136 =over 4 137 138 =item * 139 140 Specify pre-requisites in Makefile.PL or Build.PL 141 142 =item * 143 144 Specify Perl version requirements with C<use> 145 146 =item * 147 148 Include tests with your module 149 150 =item * 151 152 Choose a sensible and consistent version numbering scheme (X.YY is the common Perl module numbering scheme) 153 154 =item * 155 156 Increment the version number for every change, no matter how small 157 158 =item * 159 160 Package the module using "make dist" 161 162 =item * 163 164 Choose an appropriate license (GPL/Artistic is a good default) 165 166 =back 167 168 =head1 BEFORE YOU START WRITING A MODULE 169 170 Try not to launch headlong into developing your module without spending 171 some time thinking first. A little forethought may save you a vast 172 amount of effort later on. 173 174 =head2 Has it been done before? 175 176 You may not even need to write the module. Check whether it's already 177 been done in Perl, and avoid re-inventing the wheel unless you have a 178 good reason. 179 180 Good places to look for pre-existing modules include 181 http://search.cpan.org/ and asking on modules@perl.org 182 183 If an existing module B<almost> does what you want, consider writing a 184 patch, writing a subclass, or otherwise extending the existing module 185 rather than rewriting it. 186 187 =head2 Do one thing and do it well 188 189 At the risk of stating the obvious, modules are intended to be modular. 190 A Perl developer should be able to use modules to put together the 191 building blocks of their application. However, it's important that the 192 blocks are the right shape, and that the developer shouldn't have to use 193 a big block when all they need is a small one. 194 195 Your module should have a clearly defined scope which is no longer than 196 a single sentence. Can your module be broken down into a family of 197 related modules? 198 199 Bad example: 200 201 "FooBar.pm provides an implementation of the FOO protocol and the 202 related BAR standard." 203 204 Good example: 205 206 "Foo.pm provides an implementation of the FOO protocol. Bar.pm 207 implements the related BAR protocol." 208 209 This means that if a developer only needs a module for the BAR standard, 210 they should not be forced to install libraries for FOO as well. 211 212 =head2 What's in a name? 213 214 Make sure you choose an appropriate name for your module early on. This 215 will help people find and remember your module, and make programming 216 with your module more intuitive. 217 218 When naming your module, consider the following: 219 220 =over 4 221 222 =item * 223 224 Be descriptive (i.e. accurately describes the purpose of the module). 225 226 =item * 227 228 Be consistent with existing modules. 229 230 =item * 231 232 Reflect the functionality of the module, not the implementation. 233 234 =item * 235 236 Avoid starting a new top-level hierarchy, especially if a suitable 237 hierarchy already exists under which you could place your module. 238 239 =back 240 241 You should contact modules@perl.org to ask them about your module name 242 before publishing your module. You should also try to ask people who 243 are already familiar with the module's application domain and the CPAN 244 naming system. Authors of similar modules, or modules with similar 245 names, may be a good place to start. 246 247 =head1 DESIGNING AND WRITING YOUR MODULE 248 249 Considerations for module design and coding: 250 251 =head2 To OO or not to OO? 252 253 Your module may be object oriented (OO) or not, or it may have both kinds 254 of interfaces available. There are pros and cons of each technique, which 255 should be considered when you design your API. 256 257 According to Damian Conway, you should consider using OO: 258 259 =over 4 260 261 =item * 262 263 When the system is large or likely to become so 264 265 =item * 266 267 When the data is aggregated in obvious structures that will become objects 268 269 =item * 270 271 When the types of data form a natural hierarchy that can make use of inheritance 272 273 =item * 274 275 When operations on data vary according to data type (making 276 polymorphic invocation of methods feasible) 277 278 =item * 279 280 When it is likely that new data types may be later introduced 281 into the system, and will need to be handled by existing code 282 283 =item * 284 285 When interactions between data are best represented by 286 overloaded operators 287 288 =item * 289 290 When the implementation of system components is likely to 291 change over time (and hence should be encapsulated) 292 293 =item * 294 295 When the system design is itself object-oriented 296 297 =item * 298 299 When large amounts of client code will use the software (and 300 should be insulated from changes in its implementation) 301 302 =item * 303 304 When many separate operations will need to be applied to the 305 same set of data 306 307 =back 308 309 Think carefully about whether OO is appropriate for your module. 310 Gratuitous object orientation results in complex APIs which are 311 difficult for the average module user to understand or use. 312 313 =head2 Designing your API 314 315 Your interfaces should be understandable by an average Perl programmer. 316 The following guidelines may help you judge whether your API is 317 sufficiently straightforward: 318 319 =over 4 320 321 =item Write simple routines to do simple things. 322 323 It's better to have numerous simple routines than a few monolithic ones. 324 If your routine changes its behaviour significantly based on its 325 arguments, it's a sign that you should have two (or more) separate 326 routines. 327 328 =item Separate functionality from output. 329 330 Return your results in the most generic form possible and allow the user 331 to choose how to use them. The most generic form possible is usually a 332 Perl data structure which can then be used to generate a text report, 333 HTML, XML, a database query, or whatever else your users require. 334 335 If your routine iterates through some kind of list (such as a list of 336 files, or records in a database) you may consider providing a callback 337 so that users can manipulate each element of the list in turn. 338 File::Find provides an example of this with its 339 C<find(\&wanted, $dir)> syntax. 340 341 =item Provide sensible shortcuts and defaults. 342 343 Don't require every module user to jump through the same hoops to achieve a 344 simple result. You can always include optional parameters or routines for 345 more complex or non-standard behaviour. If most of your users have to 346 type a few almost identical lines of code when they start using your 347 module, it's a sign that you should have made that behaviour a default. 348 Another good indicator that you should use defaults is if most of your 349 users call your routines with the same arguments. 350 351 =item Naming conventions 352 353 Your naming should be consistent. For instance, it's better to have: 354 355 display_day(); 356 display_week(); 357 display_year(); 358 359 than 360 361 display_day(); 362 week_display(); 363 show_year(); 364 365 This applies equally to method names, parameter names, and anything else 366 which is visible to the user (and most things that aren't!) 367 368 =item Parameter passing 369 370 Use named parameters. It's easier to use a hash like this: 371 372 $obj->do_something( 373 name => "wibble", 374 type => "text", 375 size => 1024, 376 ); 377 378 ... than to have a long list of unnamed parameters like this: 379 380 $obj->do_something("wibble", "text", 1024); 381 382 While the list of arguments might work fine for one, two or even three 383 arguments, any more arguments become hard for the module user to 384 remember, and hard for the module author to manage. If you want to add 385 a new parameter you will have to add it to the end of the list for 386 backward compatibility, and this will probably make your list order 387 unintuitive. Also, if many elements may be undefined you may see the 388 following unattractive method calls: 389 390 $obj->do_something(undef, undef, undef, undef, undef, undef, 1024); 391 392 Provide sensible defaults for parameters which have them. Don't make 393 your users specify parameters which will almost always be the same. 394 395 The issue of whether to pass the arguments in a hash or a hashref is 396 largely a matter of personal style. 397 398 The use of hash keys starting with a hyphen (C<-name>) or entirely in 399 upper case (C<NAME>) is a relic of older versions of Perl in which 400 ordinary lower case strings were not handled correctly by the C<=E<gt>> 401 operator. While some modules retain uppercase or hyphenated argument 402 keys for historical reasons or as a matter of personal style, most new 403 modules should use simple lower case keys. Whatever you choose, be 404 consistent! 405 406 =back 407 408 =head2 Strictness and warnings 409 410 Your module should run successfully under the strict pragma and should 411 run without generating any warnings. Your module should also handle 412 taint-checking where appropriate, though this can cause difficulties in 413 many cases. 414 415 =head2 Backwards compatibility 416 417 Modules which are "stable" should not break backwards compatibility 418 without at least a long transition phase and a major change in version 419 number. 420 421 =head2 Error handling and messages 422 423 When your module encounters an error it should do one or more of: 424 425 =over 4 426 427 =item * 428 429 Return an undefined value. 430 431 =item * 432 433 set C<$Module::errstr> or similar (C<errstr> is a common name used by 434 DBI and other popular modules; if you choose something else, be sure to 435 document it clearly). 436 437 =item * 438 439 C<warn()> or C<carp()> a message to STDERR. 440 441 =item * 442 443 C<croak()> only when your module absolutely cannot figure out what to 444 do. (C<croak()> is a better version of C<die()> for use within 445 modules, which reports its errors from the perspective of the caller. 446 See L<Carp> for details of C<croak()>, C<carp()> and other useful 447 routines.) 448 449 =item * 450 451 As an alternative to the above, you may prefer to throw exceptions using 452 the Error module. 453 454 =back 455 456 Configurable error handling can be very useful to your users. Consider 457 offering a choice of levels for warning and debug messages, an option to 458 send messages to a separate file, a way to specify an error-handling 459 routine, or other such features. Be sure to default all these options 460 to the commonest use. 461 462 =head1 DOCUMENTING YOUR MODULE 463 464 =head2 POD 465 466 Your module should include documentation aimed at Perl developers. 467 You should use Perl's "plain old documentation" (POD) for your general 468 technical documentation, though you may wish to write additional 469 documentation (white papers, tutorials, etc) in some other format. 470 You need to cover the following subjects: 471 472 =over 4 473 474 =item * 475 476 A synopsis of the common uses of the module 477 478 =item * 479 480 The purpose, scope and target applications of your module 481 482 =item * 483 484 Use of each publically accessible method or subroutine, including 485 parameters and return values 486 487 =item * 488 489 Examples of use 490 491 =item * 492 493 Sources of further information 494 495 =item * 496 497 A contact email address for the author/maintainer 498 499 =back 500 501 The level of detail in Perl module documentation generally goes from 502 less detailed to more detailed. Your SYNOPSIS section should contain a 503 minimal example of use (perhaps as little as one line of code; skip the 504 unusual use cases or anything not needed by most users); the 505 DESCRIPTION should describe your module in broad terms, generally in 506 just a few paragraphs; more detail of the module's routines or methods, 507 lengthy code examples, or other in-depth material should be given in 508 subsequent sections. 509 510 Ideally, someone who's slightly familiar with your module should be able 511 to refresh their memory without hitting "page down". As your reader 512 continues through the document, they should receive a progressively 513 greater amount of knowledge. 514 515 The recommended order of sections in Perl module documentation is: 516 517 =over 4 518 519 =item * 520 521 NAME 522 523 =item * 524 525 SYNOPSIS 526 527 =item * 528 529 DESCRIPTION 530 531 =item * 532 533 One or more sections or subsections giving greater detail of available 534 methods and routines and any other relevant information. 535 536 =item * 537 538 BUGS/CAVEATS/etc 539 540 =item * 541 542 AUTHOR 543 544 =item * 545 546 SEE ALSO 547 548 =item * 549 550 COPYRIGHT and LICENSE 551 552 =back 553 554 Keep your documentation near the code it documents ("inline" 555 documentation). Include POD for a given method right above that 556 method's subroutine. This makes it easier to keep the documentation up 557 to date, and avoids having to document each piece of code twice (once in 558 POD and once in comments). 559 560 =head2 README, INSTALL, release notes, changelogs 561 562 Your module should also include a README file describing the module and 563 giving pointers to further information (website, author email). 564 565 An INSTALL file should be included, and should contain simple installation 566 instructions. When using ExtUtils::MakeMaker this will usually be: 567 568 =over 4 569 570 =item perl Makefile.PL 571 572 =item make 573 574 =item make test 575 576 =item make install 577 578 =back 579 580 When using Module::Build, this will usually be: 581 582 =over 4 583 584 =item perl Build.PL 585 586 =item perl Build 587 588 =item perl Build test 589 590 =item perl Build install 591 592 =back 593 594 Release notes or changelogs should be produced for each release of your 595 software describing user-visible changes to your module, in terms 596 relevant to the user. 597 598 =head1 RELEASE CONSIDERATIONS 599 600 =head2 Version numbering 601 602 Version numbers should indicate at least major and minor releases, and 603 possibly sub-minor releases. A major release is one in which most of 604 the functionality has changed, or in which major new functionality is 605 added. A minor release is one in which a small amount of functionality 606 has been added or changed. Sub-minor version numbers are usually used 607 for changes which do not affect functionality, such as documentation 608 patches. 609 610 The most common CPAN version numbering scheme looks like this: 611 612 1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32 613 614 A correct CPAN version number is a floating point number with at least 615 2 digits after the decimal. You can test whether it conforms to CPAN by 616 using 617 618 perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm' 619 620 If you want to release a 'beta' or 'alpha' version of a module but 621 don't want CPAN.pm to list it as most recent use an '_' after the 622 regular version number followed by at least 2 digits, eg. 1.20_01. If 623 you do this, the following idiom is recommended: 624 625 $VERSION = "1.12_01"; 626 $XS_VERSION = $VERSION; # only needed if you have XS code 627 $VERSION = eval $VERSION; 628 629 With that trick MakeMaker will only read the first line and thus read 630 the underscore, while the perl interpreter will evaluate the $VERSION 631 and convert the string into a number. Later operations that treat 632 $VERSION as a number will then be able to do so without provoking a 633 warning about $VERSION not being a number. 634 635 Never release anything (even a one-word documentation patch) without 636 incrementing the number. Even a one-word documentation patch should 637 result in a change in version at the sub-minor level. 638 639 =head2 Pre-requisites 640 641 Module authors should carefully consider whether to rely on other 642 modules, and which modules to rely on. 643 644 Most importantly, choose modules which are as stable as possible. In 645 order of preference: 646 647 =over 4 648 649 =item * 650 651 Core Perl modules 652 653 =item * 654 655 Stable CPAN modules 656 657 =item * 658 659 Unstable CPAN modules 660 661 =item * 662 663 Modules not available from CPAN 664 665 =back 666 667 Specify version requirements for other Perl modules in the 668 pre-requisites in your Makefile.PL or Build.PL. 669 670 Be sure to specify Perl version requirements both in Makefile.PL or 671 Build.PL and with C<require 5.6.1> or similar. See the section on 672 C<use VERSION> of L<perlfunc/require> for details. 673 674 =head2 Testing 675 676 All modules should be tested before distribution (using "make disttest"), 677 and the tests should also be available to people installing the modules 678 (using "make test"). 679 For Module::Build you would use the C<make test> equivalent C<perl Build test>. 680 681 The importance of these tests is proportional to the alleged stability of a 682 module -- a module which purports to be stable or which hopes to achieve wide 683 use should adhere to as strict a testing regime as possible. 684 685 Useful modules to help you write tests (with minimum impact on your 686 development process or your time) include Test::Simple, Carp::Assert 687 and Test::Inline. 688 For more sophisticated test suites there are Test::More and Test::MockObject. 689 690 =head2 Packaging 691 692 Modules should be packaged using one of the standard packaging tools. 693 Currently you have the choice between ExtUtils::MakeMaker and the 694 more platform independent Module::Build, allowing modules to be installed in a 695 consistent manner. 696 When using ExtUtils::MakeMaker, you can use "make dist" to create your 697 package. Tools exist to help you to build your module in a MakeMaker-friendly 698 style. These include ExtUtils::ModuleMaker and h2xs. See also L<perlnewmod>. 699 700 =head2 Licensing 701 702 Make sure that your module has a license, and that the full text of it 703 is included in the distribution (unless it's a common one and the terms 704 of the license don't require you to include it). 705 706 If you don't know what license to use, dual licensing under the GPL 707 and Artistic licenses (the same as Perl itself) is a good idea. 708 See L<perlgpl> and L<perlartistic>. 709 710 =head1 COMMON PITFALLS 711 712 =head2 Reinventing the wheel 713 714 There are certain application spaces which are already very, very well 715 served by CPAN. One example is templating systems, another is date and 716 time modules, and there are many more. While it is a rite of passage to 717 write your own version of these things, please consider carefully 718 whether the Perl world really needs you to publish it. 719 720 =head2 Trying to do too much 721 722 Your module will be part of a developer's toolkit. It will not, in 723 itself, form the B<entire> toolkit. It's tempting to add extra features 724 until your code is a monolithic system rather than a set of modular 725 building blocks. 726 727 =head2 Inappropriate documentation 728 729 Don't fall into the trap of writing for the wrong audience. Your 730 primary audience is a reasonably experienced developer with at least 731 a moderate understanding of your module's application domain, who's just 732 downloaded your module and wants to start using it as quickly as possible. 733 734 Tutorials, end-user documentation, research papers, FAQs etc are not 735 appropriate in a module's main documentation. If you really want to 736 write these, include them as sub-documents such as C<My::Module::Tutorial> or 737 C<My::Module::FAQ> and provide a link in the SEE ALSO section of the 738 main documentation. 739 740 =head1 SEE ALSO 741 742 =over 4 743 744 =item L<perlstyle> 745 746 General Perl style guide 747 748 =item L<perlnewmod> 749 750 How to create a new module 751 752 =item L<perlpod> 753 754 POD documentation 755 756 =item L<podchecker> 757 758 Verifies your POD's correctness 759 760 =item Packaging Tools 761 762 L<ExtUtils::MakeMaker>, L<Module::Build> 763 764 =item Testing tools 765 766 L<Test::Simple>, L<Test::Inline>, L<Carp::Assert>, L<Test::More>, L<Test::MockObject> 767 768 =item http://pause.perl.org/ 769 770 Perl Authors Upload Server. Contains links to information for module 771 authors. 772 773 =item Any good book on software engineering 774 775 =back 776 777 =head1 AUTHOR 778 779 Kirrily "Skud" Robert <skud@cpan.org> 780
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