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 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 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 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 satisfactory.

              [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 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 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 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)