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Diffstat (limited to 'pod/perlmod.pod')
| -rw-r--r-- | pod/perlmod.pod | 204 |
1 files changed, 110 insertions, 94 deletions
diff --git a/pod/perlmod.pod b/pod/perlmod.pod index 48ebf23711..de94e71ea2 100644 --- a/pod/perlmod.pod +++ b/pod/perlmod.pod @@ -6,25 +6,27 @@ perlmod - Perl modules (packages and symbol tables) =head2 Packages -Perl provides a mechanism for alternative namespaces to protect packages -from stomping on each other's variables. In fact, there's really no such -thing as a global variable in Perl (although some identifiers default -to the main package instead of the current one). The package statement -declares the compilation unit as -being in the given namespace. The scope of the package declaration -is from the declaration itself through the end of the enclosing block, -C<eval>, C<sub>, or end of file, whichever comes first (the same scope -as the my() and local() operators). All further unqualified dynamic -identifiers will be in this namespace. A package statement only affects -dynamic variables--including those you've used local() on--but -I<not> lexical variables created with my(). Typically it would be -the first declaration in a file to be included by the C<require> or -C<use> operator. You can switch into a package in more than one place; -it merely influences which symbol table is used by the compiler for the -rest of that block. You can refer to variables and filehandles in other -packages by prefixing the identifier with the package name and a double -colon: C<$Package::Variable>. If the package name is null, the C<main> -package is assumed. That is, C<$::sail> is equivalent to C<$main::sail>. +Perl provides a mechanism for alternative namespaces to protect +packages from stomping on each other's variables. In fact, there's +really no such thing as a global variable in Perl . The package +statement declares the compilation unit as being in the given +namespace. The scope of the package declaration is from the +declaration itself through the end of the enclosing block, C<eval>, +or file, whichever comes first (the same scope as the my() and +local() operators). Unqualified dynamic identifiers will be in +this namespace, except for those few identifiers that if unqualified, +default to the main package instead of the current one as described +below. A package statement affects only dynamic variables--including +those you've used local() on--but I<not> lexical variables created +with my(). Typically it would be the first declaration in a file +included by the C<do>, C<require>, or C<use> operators. You can +switch into a package in more than one place; it merely influences +which symbol table is used by the compiler for the rest of that +block. You can refer to variables and filehandles in other packages +by prefixing the identifier with the package name and a double +colon: C<$Package::Variable>. If the package name is null, the +C<main> package is assumed. That is, C<$::sail> is equivalent to +C<$main::sail>. The old package delimiter was a single quote, but double colon is now the preferred delimiter, in part because it's more readable to humans, and @@ -37,35 +39,38 @@ C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is, the $s variable in package C<owner>, which is probably not what you meant. Use braces to disambiguate, as in C<"This is ${owner}'s house">. -Packages may be nested inside other packages: C<$OUTER::INNER::var>. This -implies nothing about the order of name lookups, however. All symbols +Packages may themselves contain package separators, as in +C<$OUTER::INNER::var>. This implies nothing about the order of +name lookups, however. There are no relative packages: all symbols are either local to the current package, or must be fully qualified from the outer package name down. For instance, there is nowhere -within package C<OUTER> that C<$INNER::var> refers to C<$OUTER::INNER::var>. -It would treat package C<INNER> as a totally separate global package. - -Only identifiers starting with letters (or underscore) are stored in a -package's symbol table. All other symbols are kept in package C<main>, -including all of the punctuation variables like $_. In addition, when -unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, -INC, and SIG are forced to be in package C<main>, even when used for other -purposes than their builtin one. Note also that, if you have a package -called C<m>, C<s>, or C<y>, then you can't use the qualified form of an -identifier because it will be interpreted instead as a pattern match, -a substitution, or a transliteration. - -(Variables beginning with underscore used to be forced into package +within package C<OUTER> that C<$INNER::var> refers to +C<$OUTER::INNER::var>. It would treat package C<INNER> as a totally +separate global package. + +Only identifiers starting with letters (or underscore) are stored +in a package's symbol table. All other symbols are kept in package +C<main>, including all punctuation variables, like $_. In addition, +when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, +ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>, +even when used for other purposes than their built-in one. If you +have a package called C<m>, C<s>, or C<y>, then you can't use the +qualified form of an identifier because it would be instead interpreted +as a pattern match, a substitution, or a transliteration. + +Variables beginning with underscore used to be forced into package main, but we decided it was more useful for package writers to be able to use leading underscore to indicate private variables and method names. -$_ is still global though.) +$_ is still global though. See also L<perlvar/"Technical Note on the +Syntax of Variable Names">. -Eval()ed strings are compiled in the package in which the eval() was +C<eval>ed strings are compiled in the package in which the eval() was compiled. (Assignments to C<$SIG{}>, however, assume the signal handler specified is in the C<main> package. Qualify the signal handler name if you wish to have a signal handler in a package.) For an example, examine F<perldb.pl> in the Perl library. It initially switches to the C<DB> package so that the debugger doesn't interfere with variables -in the script you are trying to debug. At various points, however, it +in the program you are trying to debug. At various points, however, it temporarily switches back to the C<main> package to evaluate various expressions in the context of the C<main> package (or wherever you came from). See L<perldebug>. @@ -92,8 +97,8 @@ table lookups at compile time: local $main::{foo} = $main::{bar}; You can use this to print out all the variables in a package, for -instance. The standard F<dumpvar.pl> library and the CPAN module -Devel::Symdump make use of this. +instance. The standard but antequated F<dumpvar.pl> library and +the CPAN module Devel::Symdump make use of this. Assignment to a typeglob performs an aliasing operation, i.e., @@ -102,7 +107,7 @@ Assignment to a typeglob performs an aliasing operation, i.e., causes variables, subroutines, formats, and file and directory handles accessible via the identifier C<richard> also to be accessible via the identifier C<dick>. If you want to alias only a particular variable or -subroutine, you can assign a reference instead: +subroutine, assign a reference instead: *dick = \$richard; @@ -130,7 +135,7 @@ is a somewhat tricky way of passing around references cheaply when you won't want to have to remember to dereference variables explicitly. -Another use of symbol tables is for making "constant" scalars. +Another use of symbol tables is for making "constant" scalars. *PI = \3.14159265358979; @@ -157,14 +162,14 @@ This prints You gave me main::foo You gave me bar::baz -The *foo{THING} notation can also be used to obtain references to the +The C<*foo{THING}> notation can also be used to obtain references to the individual elements of *foo, see L<perlref>. =head2 Package Constructors and Destructors There are two special subroutine definitions that function as package -constructors and destructors. These are the C<BEGIN> and C<END> -routines. The C<sub> is optional for these routines. +constructors and destructors. These are the C<BEGIN>, C<INIT>, and +C<END> routines. The C<sub> is optional for these routines. A C<BEGIN> subroutine is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file @@ -183,39 +188,45 @@ trap that yourself (if you can).) You may have multiple C<END> blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO). -Inside an C<END> subroutine, C<$?> contains the value that the script is +Inside an C<END> subroutine, C<$?> contains the value that the program is going to pass to C<exit()>. You can modify C<$?> to change the exit -value of the script. Beware of changing C<$?> by accident (e.g. by +value of the program. Beware of changing C<$?> by accident (e.g. by running something via C<system>). -Note that when you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and +When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and C<END> work just as they do in B<awk>, as a degenerate case. As currently implemented (and subject to change, since its inconvenient at best), -both C<BEGIN> I<and> C<END> blocks are run when you use the B<-c> switch +both C<BEGIN> and<END> blocks are run when you use the B<-c> switch for a compile-only syntax check, although your main code is not. +Similar to C<BEGIN> blocks, C<INIT> blocks are run just before the +Perl runtime begins execution. For example, the code generators +documented in L<perlcc> make use of C<INIT> blocks to initialize +and resolve pointers to XSUBs. + =head2 Perl Classes -There is no special class syntax in Perl, but a package may function +There is no special class syntax in Perl, but a package may act as a class if it provides subroutines to act as methods. Such a package may also derive some of its methods from another class (package) -by listing the other package name in its global @ISA array (which +by listing the other package name(s) in its global @ISA array (which must be a package global, not a lexical). For more on this, see L<perltoot> and L<perlobj>. =head2 Perl Modules -A module is just a package that is defined in a library file of -the same name, and is designed to be reusable. It may do this by -providing a mechanism for exporting some of its symbols into the symbol -table of any package using it. Or it may function as a class -definition and make its semantics available implicitly through method -calls on the class and its objects, without explicit exportation of any -symbols. Or it can do a little of both. +A module is just a set of related function in a library file a Perl +package with the same name as the file. It is specifically designed +to be reusable by other modules or programs. It may do this by +providing a mechanism for exporting some of its symbols into the +symbol table of any package using it. Or it may function as a class +definition and make its semantics available implicitly through +method calls on the class and its objects, without explicitly +exportating anything. Or it can do a little of both. -For example, to start a normal module called Some::Module, create -a file called Some/Module.pm and start with this template: +For example, to start a traditional, non-OO module called Some::Module, +create a file called F<Some/Module.pm> and start with this template: package Some::Module; # assumes Some/Module.pm @@ -275,10 +286,13 @@ a file called Some/Module.pm and start with this template: END { } # module clean-up code here (global destructor) -Then go on to declare and use your variables in functions -without any qualifications. -See L<Exporter> and the L<perlmodlib> for details on -mechanics and style issues in module creation. + ## YOUR CODE GOES HERE + + 1; # don't forget to return a true value from the file + +Then go on to declare and use your variables in functions without +any qualifications. See L<Exporter> and the L<perlmodlib> for +details on mechanics and style issues in module creation. Perl modules are included into your program by saying @@ -304,12 +318,13 @@ is exactly equivalent to BEGIN { require Module; } -All Perl module files have the extension F<.pm>. C<use> assumes this so -that you don't have to spell out "F<Module.pm>" in quotes. This also -helps to differentiate new modules from old F<.pl> and F<.ph> files. -Module names are also capitalized unless they're functioning as pragmas, -"Pragmas" are in effect compiler directives, and are sometimes called -"pragmatic modules" (or even "pragmata" if you're a classicist). +All Perl module files have the extension F<.pm>. The C<use> operator +assumes this so you don't have to spell out "F<Module.pm>" in quotes. +This also helps to differentiate new modules from old F<.pl> and +F<.ph> files. Module names are also capitalized unless they're +functioning as pragmas; pragmas are in effect compiler directives, +and are sometimes called "pragmatic modules" (or even "pragmata" +if you're a classicist). The two statements: @@ -319,18 +334,19 @@ The two statements: differ from each other in two ways. In the first case, any double colons in the module name, such as C<Some::Module>, are translated into your system's directory separator, usually "/". The second -case does not, and would have to be specified literally. The other difference -is that seeing the first C<require> clues in the compiler that uses of -indirect object notation involving "SomeModule", as in C<$ob = purge SomeModule>, -are method calls, not function calls. (Yes, this really can make a difference.) - -Because the C<use> statement implies a C<BEGIN> block, the importation -of semantics happens at the moment the C<use> statement is compiled, +case does not, and would have to be specified literally. The other +difference is that seeing the first C<require> clues in the compiler +that uses of indirect object notation involving "SomeModule", as +in C<$ob = purge SomeModule>, are method calls, not function calls. +(Yes, this really can make a difference.) + +Because the C<use> statement implies a C<BEGIN> block, the importing +of semantics happens as soon as the C<use> statement is compiled, before the rest of the file is compiled. This is how it is able to function as a pragma mechanism, and also how modules are able to -declare subroutines that are then visible as list operators for +declare subroutines that are then visible as list or unary operators for the rest of the current file. This will not work if you use C<require> -instead of C<use>. With require you can get into this problem: +instead of C<use>. With C<require> you can get into this problem: require Cwd; # make Cwd:: accessible $here = Cwd::getcwd(); @@ -354,22 +370,22 @@ filenames on some systems. Therefore, if a module's name is, say, C<Text::Soundex>, then its definition is actually found in the library file F<Text/Soundex.pm>. -Perl modules always have a F<.pm> file, but there may also be dynamically -linked executables or autoloaded subroutine definitions associated with -the module. If so, these will be entirely transparent to the user of -the module. It is the responsibility of the F<.pm> file to load (or -arrange to autoload) any additional functionality. The POSIX module -happens to do both dynamic loading and autoloading, but the user can -say just C<use POSIX> to get it all. - -For more information on writing extension modules, see L<perlxstut> -and L<perlguts>. +Perl modules always have a F<.pm> file, but there may also be +dynamically linked executables (often ending in F<.so>) or autoloaded +subroutine definitions (often ending in F<.al> associated with the +module. If so, these will be entirely transparent to the user of +the module. It is the responsibility of the F<.pm> file to load +(or arrange to autoload) any additional functionality. For example, +although the POSIX module happens to do both dynamic loading and +autoloading, but the user can say just C<use POSIX> to get it all. =head1 SEE ALSO See L<perlmodlib> for general style issues related to building Perl -modules and classes as well as descriptions of the standard library and -CPAN, L<Exporter> for how Perl's standard import/export mechanism works, -L<perltoot> for an in-depth tutorial on creating classes, L<perlobj> -for a hard-core reference document on objects, and L<perlsub> for an -explanation of functions and scoping. +modules and classes, as well as descriptions of the standard library +and CPAN, L<Exporter> for how Perl's standard import/export mechanism +works, L<perltoot> and L<perltootc> for an in-depth tutorial on +creating classes, L<perlobj> for a hard-core reference document on +objects, L<perlsub> for an explanation of functions and scoping, +and L<perlxstut> and L<perlguts> for more information on writing +extension modules. |