Provided by: tcl8.4-doc_8.4.20-8_all 
NAME
package - Facilities for package loading and version control
SYNOPSIS
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
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DESCRIPTION
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 ?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 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 ?-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 acceptable as long as they have the same major version number as version. 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 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 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 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 version number is
supplied to the package require command, then the version 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 version2 and they both have the same major
version number), 0 otherwise.
VERSION NUMBERS
Version numbers consist of one or more decimal numbers separated by dots, such as 2 or
1.162 or 3.1.13.1. 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 1.3.0.0, so it is earlier than 1.3.1 or 1.3.0.2. 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.
PACKAGE INDICES
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.
EXAMPLES
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
}
SEE ALSO
msgcat(3tcl), packagens(3tcl), pkgMkIndex(3tcl)
KEYWORDS
package, version