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Text File | 1995-07-26 | 25.0 KB | 595 lines

PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) NNNNAAAAMMMMEEEE perlmod - Perl modules (packages) DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN PPPPaaaacccckkkkaaaaggggeeeessss Perl provides a mechanism for alternate namespaces to protect packages from stomping on each others variables. By default, a Perl script starts compiling into the package known as main. You can switch namespaces using the package declaration. The scope of the package declaration is from the declaration itself to the end of the enclosing block (the same scope as the _l_o_c_a_l() operator). Typically it would be the first declaration in a file to be included by the require 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: $Package::Variable. If the package name is null, the main package as assumed. That is, $::sail is equivalent to $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 in part because it's more readable to eeeemmmmaaaaccccssss macros. It also makes C++ programmers feel like they know what's going on.) Packages may be nested inside other packages: $OUTER::INNER::var. This implies nothing about the order of name lookups, however. 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 OUTER that $INNER::var refers to $OUTER::INNER::var. It would treat package 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 main. In addition, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are forced to be in package main, even when used for other purposes than their built-in one. Note also that, if you have a package called m, s or y, then you can't use the qualified form of an identifier since it will be interpreted instead as a pattern match, a substitution, or a translation. (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.) Page 1 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) _E_v_a_l()ed strings are compiled in the package in which the _e_v_a_l() was compiled. (Assignments to $SIG{}, however, assume the signal handler specified is in the C<main. package. Qualify the signal handler name if you wish to have a signal handler in a package.) For an example, examine _p_e_r_l_d_b._p_l in the Perl library. It initially switches to the DB package so that the debugger doesn't interfere with variables in the script you are trying to debug. At various points, however, it temporarily switches back to the main package to evaluate various expressions in the context of the main package (or wherever you came from). See the _p_e_r_l_d_e_b_u_g manpage. SSSSyyyymmmmbbbboooollll TTTTaaaabbbblllleeeessss The symbol table for a package happens to be stored in the associative array of that name appended with two colons. The main symbol table's name is thus %main::, or %:: for short. Likewise the nested package mentioned earlier is named %OUTER::INNER::. The value in each entry of the associative array is what you are referring to when you use the *name notation. In fact, the following have the same effect, though the first is more efficient because it does the symbol table lookups at compile time: local(*main::foo) = *main::bar; local($main::{'foo'}) = $main::{'bar'}; You can use this to print out all the variables in a package, for instance. Here is _d_u_m_p_v_a_r._p_l from the Perl library: package dumpvar; sub main::dumpvar { ($package) = @_; local(*stab) = eval("*${package}::"); while (($key,$val) = each(%stab)) { local(*entry) = $val; if (defined $entry) { print "\$$key = '$entry'\n"; } if (defined @entry) { print "\@$key = (\n"; foreach $num ($[ .. $#entry) { print " $num\t'",$entry[$num],"'\n"; } print ")\n"; } Page 2 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) if ($key ne "${package}::" && defined %entry) { print "\%$key = (\n"; foreach $key (sort keys(%entry)) { print " $key\t'",$entry{$key},"'\n"; } print ")\n"; } } } Note that even though the subroutine is compiled in package dumpvar, the name of the subroutine is qualified so that its name is inserted into package main. Assignment to a symbol table entry performs an aliasing operation, i.e., *dick = *richard; causes variables, subroutines and filehandles accessible via the identifier richard to also be accessible via the symbol dick. If you only want to alias a particular variable or subroutine, you can assign a reference instead: *dick = \$richard; makes $richard and $dick the same variable, but leaves @richard and @dick as separate arrays. Tricky, eh? PPPPaaaacccckkkkaaaaggggeeee CCCCoooonnnnssssttttrrrruuuuccccttttoooorrrrssss aaaannnndddd DDDDeeeessssttttrrrruuuuccccttttoooorrrrssss There are two special subroutine definitions that function as package constructors and destructors. These are the BEGIN and END routines. The sub is optional for these routines. A BEGIN subroutine is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file is parsed. You may have multiple BEGIN blocks within a file--they will execute in order of definition. Because a BEGIN block executes immediately, it can pull in definitions of subroutines and such from other files in time to be visible to the rest of the file. An END subroutine is executed as late as possible, that is, when the interpreter is being exited, even if it is exiting as a result of a _d_i_e() function. (But not if it's is being blown out of the water by a signal--you have to trap that yourself (if you can).) You may have multiple END blocks within a file--they wil execute in reverse order of definition; that is: last in, first out (LIFO). Page 3 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) Note that when you use the ----nnnn and ----pppp switches to Perl, BEGIN and END work just as they do in aaaawwwwkkkk, as a degenerate case. PPPPeeeerrrrllll CCCCllllaaaasssssssseeeessss There is no special class syntax in Perl 5, but a package may function as a class if it provides subroutines that function as methods. Such a package may also derive some of its methods from another class package by listing the other package name in its @ISA array. For more on this, see the _p_e_r_l_o_b_j manpage. PPPPeeeerrrrllll MMMMoooodddduuuulllleeeessss In Perl 5, the notion of packages has been extended into the notion of modules. A module is 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. Perl modules are included by saying use Module; or use Module LIST; This is exactly equivalent to BEGIN { require "Module.pm"; import Module; } or BEGIN { require "Module.pm"; import Module LIST; } All Perl module files have the extension ._p_m. use assumes this so that you don't have to spell out "_M_o_d_u_l_e._p_m" in quotes. This also helps to differentiate new modules from old ._p_l and ._p_h 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). Because the use statement implies a BEGIN block, the importation of semantics happens at the moment the use statement is compiled, before the rest of the file is Page 4 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) 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 the rest of the current file. This will not work if you use require instead of use. Therefore, if you're planning on the module altering your namespace, use use; otherwise, use require. Otherwise you can get into this problem: require Cwd; # make Cwd:: accessible $here = Cwd::getcwd(); use Cwd; # import names from Cwd:: $here = getcwd(); require Cwd; # make Cwd:: accessible $here = getcwd(); # oops! no main::getcwd() Perl packages may be nested inside other package names, so we can have package names containing ::. But if we used that package name directly as a filename it would makes for unwieldy or impossible filenames on some systems. Therefore, if a module's name is, say, Text::Soundex, then its definition is actually found in the library file _T_e_x_t/_S_o_u_n_d_e_x._p_m. Perl modules always have a ._p_m 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 ._p_m 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 just say use POSIX to get it all. For more information on writing extension modules, see the _p_e_r_l_a_p_i manpage and the _p_e_r_l_g_u_t_s manpage. NNNNOOOOTTTTEEEE Perl does not enforce private and public parts of its modules as you may have been used to in other languages like C++, Ada, or Modula-17. Perl doesn't have an infatuation with enforced privacy. It would prefer that you stayed out of its living room because you weren't invited, not because it has a shotgun. The module and its user have a contract, part of which is common law, and part of which is "written". Part of the common law contract is that a module doesn't pollute any namespace it wasn't asked to. The written contract for the module (AKA documentation) may make other provisions. But then you know when you use RedefineTheWorld that you're redefining the world and willing to take the consequences. Page 5 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) TTTTHHHHEEEE PPPPEEEERRRRLLLL MMMMOOOODDDDUUUULLLLEEEE LLLLIIIIBBBBRRRRAAAARRRRYYYY A number of modules are included the the Perl distribution. These are described below, and all end in ._p_m. You may also discover files in the library directory that end in either ._p_l or ._p_h. These are old libaries supplied so that old programs that use them still run. The the ._p_h files made by hhhh2222pppphhhh will probably end up as extension modules made by hhhh2222xxxxssss. (Some ._p_h values may already be available through the POSIX module.) The ppppllll2222ppppmmmm file in the distribution may help in your conversion, but it's just a mechanical process, so is far from bullet proof. PPPPrrrraaaaggggmmmmaaaattttiiiicccc MMMMoooodddduuuulllleeeessss They work somewhat like pragmas in that they tend to affect the compilation of your program, and thus will usually only work well when used within a use, or no. These are locally scoped, so if an inner BLOCK may countermand any of these by saying no integer; no strict 'refs'; which lasts until the end of that BLOCK. The following programs are defined (and have their own documentation). integer Perl pragma to compute arithmetic in integer instead of double less Perl pragma to request less of something from the compiler sigtrap Perl pragma to enable stack backtrace on unexpected signals strict Perl pragma to restrict unsafe constructs subs Perl pragma to predeclare sub names SSSSttttaaaannnnddddaaaarrrrdddd MMMMoooodddduuuulllleeeessss The following modules are all expacted to behave in a well- defined manner with respect to namespace pollution because they use the Exporter module. See their own documentation for details. Abbrev create an abbreviation table from a list AnyDBM_File provide framework for multiple DBMs Page 6 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) AutoLoader load functions only on demand AutoSplit split a package for autoloading Basename parse file anme and path from a specification Benchmark benchmark running times of code Carp warn or die of errors (from perspective of caller) CheckTree run many filetest checks on a tree Collate compare 8-bit scalar data according to the current locale Config access Perl configuration option Cwd get pathname of current working directory DynaLoader Dynamically load C libraries into Perl code English use nice English (or aaaawwwwkkkk) names for ugly punctuation variables Env Perl module that imports environment variables Exporter module to control namespace manipulations Fcntl load the C Fcntl.h defines FileHandle supply object methods for filehandles Find traverse a file tree Finddepth traverse a directory structure depth-first Getopt basic and extended _g_e_t_o_p_t(3) processing MakeMaker generate a Makefile for Perl extension Open2 open a process for both reading and writing Open3 open a process for reading, writing, and error handling POSIX Perl interface to IEEE 1003.1 namespace Ping check a host for upness Socket load the C socket.h defines Page 7 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) EEEExxxxtttteeeennnnssssiiiioooonnnn MMMMoooodddduuuulllleeeessss Extension modules are written in C (or a mix of Perl and C) and get dynamically loaded into Perl if and when you need them. Supported extension modules include the Socket, Fcntl, and POSIX modules. The following are popular C extension modules, which while available at Perl 5.0 release time, do not come not bundled (at least, not completely) due to their size, volatility, or simply lack of time for adequate testing and configuration across the multitude of platforms on which Perl was beta- tested. You are encouraged to look for them in _a_r_c_h_i_e(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even their authors before randomly posting asking for their present condition and disposition. There's no guarantee that the names or addresses below have not changed since printing, and in fact, they probably have! Curses Written by William Setzer <_W_i_l_l_i_a_m__S_e_t_z_e_r@_n_c_s_u._e_d_u>, while not included with the standard distribution, this extension module ports to most systems. FTP from your nearest Perl archive site, or try ftp://ftp.ncsu.edu/pub/math/wsetzer/cursperl5??.tar.gz It is currently in alpha test, so the name and ftp location may change. DBI This is the portable database interface written by <_T_i_m._B_u_n_c_e@_i_g._c_o._u_k>. This supersedes the many perl4 ports for database extensions. The official archive for DBperl extensions is _f_t_p._d_e_m_o_n._c_o._u_k:/_p_u_b/_p_e_r_l/_d_b. This archive contains copies of perl4 ports for Ingres, Oracle, Sybase, Informix, Unify, Postgres, and Interbase, as well as rdb and shql and other non-SQL systems. DB_File Fastest and most restriction-free of the DBM bindings, this extension module uses the popular Berkeley DB to _t_i_e() into your hashes. This has a standardly-distributed man page and dynamic loading extension module, but you'll have to fetch the Berkeley code yourself. See the _D_B__F_i_l_e manpage for where. Sx This extension module is a front to the Athena and Xlib libraries for Perl GUI progamming, originally written by by Dominic Giampaolo <_d_b_g@_s_g_i._c_o_m>, then and rewritten for Sx by Page 8 (printed 6/30/95) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111)))) Frederic Chauveau <_f_m_c@_p_a_s_t_e_u_r._f_r>. It's available for FTP from ftp.pasteur.fr:/pub/Perl/Sx.tar.gz Tk This extension module is an object-oriented Perl5 binding to the popular tcl/tk X11 package. However, you need know no TCL to use it! It was written by Malcolm Beattie <_m_b_e_a_t_t_i_e@_s_a_b_l_e._o_x._a_c._u_k>. If you are unable to locate it using _a_r_c_h_i_e(1L) or a similar tool, you may try retrieving it from /_p_r_i_v_a_t_e/_T_k- _o_c_t_o_b_e_r._t_a_r._g_z from Malcolm's machine listed above. Page 9 (printed 6/30/95)