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package(n)		     Tcl Built-In Commands		    package(n)


       package - Facilities for package loading and version control

       package forget ?package package ...?
       package ifneeded package version ?script?
       package names
       package present ?-exact? package ?version?
       package provide package ?version?
       package require ?-exact? package ?version?
       package unknown ?command?
       package vcompare version1 version2
       package versions package
       package vsatisfies version1 version2

       This  command keeps a simple database of the packages available for use
       by the current interpreter and how to load them into  the  interpreter.
       It supports multiple versions of each package and arranges for the cor‐
       rect version of a package to be loaded based on what is needed  by  the
       application.   This  command  also detects and reports version clashes.
       Typically, only the package require and package	provide	 commands  are
       invoked	in  normal Tcl scripts;	 the other commands are used primarily
       by system scripts that maintain the package database.

       The behavior of the package command is determined by  its  first	 argu‐
       ment.  The following forms are permitted:

       package forget ?package package ...?
	      Removes  all  information about each specified package from this
	      interpreter, including  information  provided  by	 both  package
	      ifneeded and package provide.

       package ifneeded package version ?script?
	      This  command  typically	appears	 only  in system configuration
	      scripts to set up the package database.	It  indicates  that  a
	      particular  version  of  a  particular  package  is available if
	      needed, and that the package can be added to the interpreter  by
	      executing	 script.  The script is saved in a database for use by
	      subsequent package require commands;  typically, script sets  up
	      auto-loading  for	 the  commands	in  the package (or calls load
	      and/or source directly), then invokes package provide  to	 indi‐
	      cate  that  the package is present.  There may be information in
	      the database for several different versions of a single package.
	      If  the  database	 already  contains information for package and
	      version, the new script  replaces	 the  existing	one.   If  the
	      script  argument is omitted, the current script for version ver‐
	      sion of package package is returned, or an empty	string	if  no
	      package  ifneeded	 command has been invoked for this package and

       package names
	      Returns a list of the names of all packages in  the  interpreter
	      for  which  a version has been provided (via package provide) or
	      for which a package ifneeded script is available.	 The order  of
	      elements in the list is arbitrary.

       package present ?-exact? package ?version?
	      This  command  is	 equivalent  to package require except that it
	      does not try and load the package if it is not already loaded.

       package provide package ?version?
	      This command is invoked to  indicate  that  version  version  of
	      package  package is now present in the interpreter.  It is typi‐
	      cally invoked once as part of an ifneeded script, and  again  by
	      the  package  itself when it is finally loaded.  An error occurs
	      if a different version of package has been provided by a	previ‐
	      ous  package  provide command.  If the version argument is omit‐
	      ted, then the command returns the version number	that  is  cur‐
	      rently  provided,	 or an empty string if no package provide com‐
	      mand has been invoked for package in this interpreter.

       package require ?-exact? package ?version?
	      This command is typically invoked by Tcl code that wishes to use
	      a	 particular  version  of  a particular package.	 The arguments
	      indicate which package is wanted, and the command ensures that a
	      suitable	version of the package is loaded into the interpreter.
	      If the command succeeds, it returns the version number  that  is
	      loaded;	otherwise  it  generates an error.  If both the -exact
	      switch and the version argument  are  specified  then  only  the
	      given  version  is acceptable.  If -exact is omitted but version
	      is specified, then versions later than version are also  accept‐
	      able  as long as they have the same major version number as ver‐
	      sion.  If both -exact and version are omitted then  any  version
	      whatsoever  is  acceptable.  If a version of package has already
	      been provided (by invoking the package  provide  command),  then
	      its version number must satisfy the criteria given by -exact and
	      version and the command  returns	immediately.   Otherwise,  the
	      command  searches the database of information provided by previ‐
	      ous package ifneeded commands to see if an acceptable version of
	      the  package  is	available.   If so, the script for the highest
	      acceptable version number is evaluated in the global  namespace;
	      it  must do whatever is necessary to load the package, including
	      calling  package	provide	 for  the  package.   If  the  package
	      ifneeded	database does not contain an acceptable version of the
	      package and a package unknown command has been specified for the
	      interpreter  then that command is evaluated in the global names‐
	      pace;  when it completes, Tcl checks again to see if the package
	      is now provided or if there is a package ifneeded script for it.
	      If all of these steps fail to provide an acceptable  version  of
	      the package, then the command returns an error.

       package unknown ?command?
	      This command supplies a ``last resort'' command to invoke during
	      package require if no suitable version of a package can be found
	      in  the  package	ifneeded database.  If the command argument is
	      supplied, it contains the first part of  a  command;   when  the
	      command is invoked during a package require command, Tcl appends
	      two additional arguments giving the  desired  package  name  and
	      version.	 For example, if command is foo bar and later the com‐
	      mand package require test 2.4 is invoked, then Tcl will  execute
	      the command foo bar test 2.4 to load the package.	 If no version
	      number is supplied to the package require command, then the ver‐
	      sion  argument  for the invoked command will be an empty string.
	      If the package unknown command  is  invoked  without  a  command
	      argument,	 then  the current package unknown script is returned,
	      or an empty string if there is none.  If command is specified as
	      an  empty	 string,  then	the  current package unknown script is
	      removed, if there is one.

       package vcompare version1 version2
	      Compares the two version numbers given by version1 and version2.
	      Returns -1 if version1 is an earlier version than version2, 0 if
	      they are equal, and 1 if version1 is later than version2.

       package versions package
	      Returns a list of all the version numbers of package  for	 which
	      information has been provided by package ifneeded commands.

       package vsatisfies version1 version2
	      Returns  1  if  scripts written for version2 will work unchanged
	      with version1 (i.e. version1 is equal to or  greater  than  ver‐
	      sion2  and they both have the same major version number), 0 oth‐

       Version numbers consist of one or more  decimal	numbers	 separated  by
       dots,  such  as 2 or 1.162 or	The first number is called the
       major version number.  Larger numbers correspond to later versions of a
       package,	 with leftmost numbers having greater significance.  For exam‐
       ple, version 2.1 is later than 1.3 and  version	3.4.6  is  later  than
       3.3.5.	Missing	 fields	 are equivalent to zeroes:  version 1.3 is the
       same as version 1.3.0 and, so  it  is  earlier  than  1.3.1  or	  A  later  version number is assumed to be upwards compatible
       with an earlier version number as long as both versions have  the  same
       major version number.  For example, Tcl scripts written for version 2.3
       of a package should work	 unchanged  under  versions  2.3.2,  2.4,  and
       2.5.1.	Changes	 in  the  major	 version  number  signify incompatible
       changes: if code is written to use version 2.1 of a package, it is  not
       guaranteed to work unmodified with either version 1.7.3 or version 3.1.

       The recommended way to use packages in Tcl is to invoke package require
       and  package  provide  commands	in  scripts,  and  use	the  procedure
       pkg_mkIndex  to	create	package	 index	files.	Once you've done this,
       packages will be loaded automatically in response  to  package  require
       commands.  See the documentation for pkg_mkIndex for details.

       To  state that a Tcl script requires the Tk and http packages, put this
       at the top of the script:
	      package require Tk
	      package require http

       To test to see if the Snack package is available	 and  load  if	it  is
       (often  useful  for optional enhancements to programs where the loss of
       the functionality is not critical) do this:
	      if {[catch {package require Snack}]} {
		 # Error thrown - package not found.
		 # Set up a dummy interface to work around the absence
	      } else {
		 # We have the package, configure the app to use it

       msgcat(n), packagens(n), pkgMkIndex(n)

       package, version

Tcl				      7.5			    package(n)
                             _         _         _ 
                            | |       | |       | |     
                            | |       | |       | |     
                         __ | | __ __ | | __ __ | | __  
                         \ \| |/ / \ \| |/ / \ \| |/ /  
                          \ \ / /   \ \ / /   \ \ / /   
                           \   /     \   /     \   /    
                            \_/       \_/       \_/ 
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