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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