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       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 correct 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 argument.  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
              indicate 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 version  of  package
              package  is  returned,  or  an empty string if no package ifneeded command has been
              invoked for this package and version.

       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 typically 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 previous  package  provide
              command.   If the version argument is omitted, then the command returns the version
              number that is currently provided, or an empty string if no package provide command
              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 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.

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

              In the “stable” selection mode the command will select the highest  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
              version satisfying all the requirements, regardless of its stableness.

              If a version of package has already been provided (by invoking the package  provide
              command),  then  its  version  number must satisfy the requirements and the command
              returns immediately.  Otherwise, the command searches the database  of  information
              provided  by  previous 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 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

       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 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 version requirement...
              Returns 1 if the version satisfies at least one of the given  requirements,  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 syntax 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 requirements to be

              [2]    A version satisfies a “bounded” requirement when

                     [a]    For min equal to the max if, and only if the version is equal to  the

                     [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 version 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  “stable”,  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 “stable”, 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 argument error.

              When an interpreter is created, its initial selection mode value is set to “stable”
              unless the environment variable TCL_PKG_PREFER_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 example, 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
       semantically  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 mutually  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 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  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
              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
              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(3tcl), packagens(3tcl), pkgMkIndex(3tcl)


       package, version