WritingPerl5Modules_AnIntroduction

1、Writing Perl 5 Modules:An IntroductionKeith Arner1OutlineWhat do we mean by Modules and Classes?Module BasicsUsing Modules, Packages, Special Variables, Exporting FunctionsAdvanced ConceptsAutoloading, More Exporting, VersioningAsk questions at any time2Topics Not CoveredObjectsModules available on

2、CPANWriting modules for CPAN distributionEach of these would make a good topic for a future talk (Any volunteers?)3Modules and ClassesThere are two types of modularization under Perl 5Modules : library functionsClasses : object-oriented programming4What is a Module?A module is a .pm file that define

3、s a library of related functionsModules are conceptually similar to old-fashioned Perl libraries (.pl files), but have a cleaner implementationselective namespace clutteringsimpler function invocation5How to use a Module Time:Localuse Time:Local;$time = timelocal($sec, $min, $hours, $mday, $mon, $ye

4、ar);print Seconds since epoch: $timen;6Module BasicsLearn by Example: MyRand.pmRefer to attached printout of MyRand.pmNote that line numbers are for reference purposes only7Package Names and FilenamesPackage name is declared on line 1This should be the same as the filename, without the .pm extension

5、If it is different, your functions will not be exported correctlyShould begin with a capital letter to avoid possible conflict with pragmas8More on Package Namespackage MyRandis file MyRand.pmpackage Timeis file Time.pmpackage Time:Localis file Time/Local.pmThey have nothing to do with one another9T

6、he Package: Your Own Private NamespaceEach package is a separate namespace for functions and variablesThere is no collision of names between namespaces$seed in the package main and $seed in the package MyRand are distinct variables10Magic VariablesEXPORT, ISA and $VERSION have special meaningsEXPORT

7、 tells Perl what functions you want to make available to the publicISA tells Perl how to export the functions (I wont go into the details)$VERSION is used to indicate compatibility11Exporting FunctionsLine 11 sets EXPORT to contain a list of the names of functions to be exportedNote that we use qw()

8、 to generate the listThis is just syntactic sugar for:EXPORT = (srand, rand);12What does it mean to Export?Your functions are defined in your own packageExporting makes the functions available in another package as if they were defined in the other packageThis is done by fiddling with symbol tables

9、(which is a topic for another talk)13A Function is a Functionis a FunctionOn lines 19 and 23, functions are declared and written exactly as they would be anywhere else14Module Specific StateLine 15 declares a variable local to the module that can maintain its state between function calls$seed is vis

10、ible to all functions in this file$seed does not conflict with $seed in any other module, nor in a mainline program15Declaring VictoryAll Perl modules must return a true value to indicate that they completed successfullyLine 35 shows the canonical returnAny true value will do:true3.1415926535$O & $/

11、(3.99)16How do you use it?Make sure your module is in your include path, INCThis can be augmented with -I or your PERL5LIB environment variableuse MyRand;srand(5);print rand;17Advanced TopicsAutoloadingSelective ExportingVersioningMuch, much more18AutoloadingThe autoloader is used to improve perform

12、ance of large modules (such as POSIX.pm)Functions are split into separate files and compiled only as neededAutoSplit.pm is used to split modules19More ExportingEXPORT can export variables in addition to functions:EXPORT = qw(rand srand $seed)EXPORT_OK and %EXPORT_TAGS can be used to export selective

13、ly20Selective ExportingIt is not always desirable to export everything, so EXPORT_OK is usedEXPORT = qw (rand srand);EXPORT_OK = qw ($seed);use MyRand qw(rand $seed);21Fancy ExportingTyping all the names of symbols to import can become a hassle, so %EXPORT_TAGS is usedEXPORT = qw (rand srand);EXPORT

14、_OK = qw ($seed);%EXPORT_TAGS = (funcs = qw(rand srand),vars = qw($seed) );use MyRand qw(:funcs :vars);22Versioning$VERSION is used to indicate the version of the moduleuse MyRand 2.0;will fail if $MyRand:VERSION 2.0Can be used with RCS Keywords:$VERSION =($REVISION 1.0$ =m/Revision ($+)/);23Much, much moreOverriding builtin functionsCascading inheritance with ISAAutomated CleanupObject Oriented Programming24ReferencesProgramming Perl, Second Edition. Wall, Christiansen & SchwartzChapters 5 and 7Man pagesperlmod