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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 package ?requirement...?
       package present -exact package version
       package provide package ?version?
       package require package ?requirement...?
       package require -exact package version
       package unknown ?command?
       package vcompare	version1 version2
       package versions	package
       package vsatisfies version requirement...
       package prefer ?latest|stable?

       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	?requirement...?
	      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 package ?requirement...?
	      This command is typically	invoked	by Tcl code that wishes	to use
	      a	particular version of a	particular package.  The arguments in-
	      dicate 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.

	      A	suitable version of the	package	is any version which satisfies
	      at least one of the requirements,	per the	rules of package vsat-
	      isfies.  If  multiple  versions  are suitable the	implementation
	      with the highest version is chosen. This last part is  addition-
	      ally influenced by the selection mode set	with package prefer.

	      In the "stable" selection	mode the command will select the high-
	      est stable version satisfying the	requirements, if  any.	If  no
	      stable  version satisfies	the requirements, the highest unstable
	      version satisfying the requirements will be  selected.   In  the
	      "latest" selection mode the command will accept the highest ver-
	      sion satisfying all the requirements, regardless of its  stable-

	      If  a  version of	package	has already been provided (by invoking
	      the package provide command), then its version number must  sat-
	      isfy the requirements and	the command returns immediately.  Oth-
	      erwise, the command searches the database	 of  information  pro-
	      vided by previous	package	ifneeded commands to see if an accept-
	      able 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 pack-
	      age 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
	      namespace;   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 require -exact package version
	      This form	of the command is used when only the given version  of
	      package is acceptable to the caller.  This command is equivalent
	      to package require package version-version.

       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
	      one or more additional arguments giving the desired package name
	      and requirements.	 For example, if command is foo	bar and	 later
	      the  command  package require test 2.4 is	invoked, then Tcl will
	      execute the command foo bar test 2.4 to load the package.	 If no
	      requirements  are	 supplied to the package require command, then
	      only the name will be added to invoked command.  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

       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 version requirement...
	      Returns 1	if the version satisfies at least one of the given re-
	      quirements, and 0	otherwise. Each	requirement is allowed to have
	      any of the forms:

	      min    This form is called "min-bounded".

	      min-   This form is called "min-unbound".

		     This form is called "bounded".

	      where "min" and "max" are	valid version numbers. The legacy syn-
	      tax  is  a special case of the extended syntax, keeping backward
	      compatibility. Regarding satisfaction the	rules are:

	      [1]    The version has to	pass at	least one of  the  listed  re-
		     quirements	to be satisfactory.

	      [2]    A version satisfies a "bounded" requirement when

		     [a]    For	 min equal to the max if, and only if the ver-
			    sion is equal to the min.

		     [b]    Otherwise if, and only if the version  is  greater
			    than  or  equal to the min,	and less than the max,
			    where both min and max have	been padded internally
			    with  "a0".	 Note that while the comparison	to min
			    is inclusive, the comparison to max	is exclusive.

	      [3]    A "min-bounded" requirement is a "bounded"	requirement in
		     disguise,	with  the max part implicitly specified	as the
		     next higher major version number of the min part. A  ver-
		     sion satisfies it per the rules above.

	      [4]    A	version	 satisfies a "min-unbound" requirement if, and
		     only if it	is greater than	or equal to the	min, where the
		     min  has  been  padded internally with "a0".  There is no
		     constraint	to a maximum.

       package prefer ?latest|stable?
	      With no arguments, the commands returns either "latest" or "sta-
	      ble",  whichever	describes  the current mode of selection logic
	      used by package require.

	      When passed the argument "latest", it sets the  selection	 logic
	      mode to "latest".

	      When  passed the argument	"stable", if the mode is already "sta-
	      ble", that value is kept.	 If the	mode is	already	"latest", then
	      the  attempt  to	set it back to "stable"	is ineffective and the
	      mode value remains "latest".

	      When passed any other value as an	argument, raise	an invalid ar-
	      gument error.

	      When an interpreter is created, its initial selection mode value
	      is set to	"stable" unless	the environment	variable  TCL_PKG_PRE-
	      FER_LATEST  is  set.   If	 that  environment variable is defined
	      (with any	value) then the	initial	(and permanent)	selection mode
	      value is set to "latest".

       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	 In addition, the letters "a" (alpha) and/or  "b"  (beta)  may
       appear  exactly once to replace a dot for separation. These letters se-
       mantically add a	negative specifier into	the version, where "a" is  -2,
       and  "b"	is -1. Each may	be specified only once,	and "a"	or "b" are mu-
       tually exclusive	in a  specifier.  Thus	1.3a1  becomes	(semantically)
       1.3.-2.1,  1.3b1	is 1.3.-1.1. Negative numbers are not directly allowed
       in version specifiers.  A version number	not containing the letters "a"
       or  "b" as specified above is called a stable version, whereas presence
       of the letters causes the version to be called is  unstable.   A	 later
       version number is assumed to be upwards compatible with an earlier ver-
       sion number as long as both versions have the same major	 version  num-
       ber.   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 writ-
       ten to use version 2.1 of a package, it is not guaranteed to  work  un-
       modified	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 have 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 (of-
       ten  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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