package(n) Tcl Built-In Commands package(n)
NAME
package - Facilities for package loading and version control
SYNOPSIS
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?
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 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
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 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
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 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 selection mode the command will select the highest stable
version satisfying the requirements, if any. If no stable ver-
sion satisfies the requirements, the highest unstable version
satisfying the requirements will be selected. In the 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 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 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 con-
tains the first part of a command; when the command is invoked
during a package require command, Tcl appends one or more addi-
tional arguments giving the desired package name and require-
ments. 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 require-
ments 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- This form is called
min-max
This form is called
where and are valid version numbers. The legacy syntax is a spe-
cial case of the extended syntax, keeping backward compatibil-
ity. 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 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 Note that while the comparison to min is
inclusive, the comparison to max is exclusive.
[3] A requirement is a requirement in disguise, with the max
part implicitly specified as the next higher major ver-
sion number of the min part. A version satisfies it per
the rules above.
[4] A version satisfies a requirement if, and only if it is
greater than or equal to the min, where the min has been
padded internally with There is no constraint to a maxi-
mum.
package prefer ?latest|stable?
With no arguments, the commands returns either or whichever
describes the current mode of selection logic used by package
require.
When passed the argument it sets the selection logic mode to
When passed the argument if the mode is already that value is
kept. If the mode is already then the attempt to set it back to
is ineffective and the mode value remains
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 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
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 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 1.3.0.0, so it is earlier than 1.3.1 or
1.3.0.2. In addition, the letters (alpha) and/or (beta) may appear
exactly once to replace a dot for separation. These letters semanti-
cally add a negative specifier into the version, where is -2, and is
-1. Each may be specified only once, and or 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 speci-
fiers. A version number not containing the letters or 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.
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 have 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(n), packagens(n), pkgMkIndex(n)
KEYWORDS
package, version
Tcl 7.5 package(n)
tcl 8.6.0 - Generated Thu Jan 10 15:00:06 CST 2013