source: downloads/tcl8.5.2/doc/package.n @ 35

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'\" Copyright (c) 1996 Sun Microsystems, Inc.
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'\" RCS: @(#) $Id: package.n,v 1.22 2007/12/13 15:22:33 dgp Exp $
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.so man.macros
.TH package n 7.5 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
package \- Facilities for package loading and version control
.SH SYNOPSIS
.nf
\fBpackage forget ?\fIpackage package ...\fR?
\fBpackage ifneeded \fIpackage version\fR ?\fIscript\fR?
\fBpackage names\fR
\fBpackage present \fIpackage \fR?\fIrequirement...\fR?
\fBpackage present \-exact \fIpackage version\fR
\fBpackage provide \fIpackage \fR?\fIversion\fR?
\fBpackage require \fIpackage \fR?\fIrequirement...\fR?
\fBpackage require \-exact \fIpackage version\fR
\fBpackage unknown \fR?\fIcommand\fR?
\fBpackage vcompare \fIversion1 version2\fR
\fBpackage versions \fIpackage\fR
\fBpackage vsatisfies \fIversion requirement...\fR
\fBpackage prefer \fR?\fBlatest\fR|\fBstable\fR?
.fi
.BE

.SH DESCRIPTION
.PP
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 \fBpackage require\fR and \fBpackage provide\fR
commands are invoked in normal Tcl scripts;  the other commands are used
primarily by system scripts that maintain the package database.
.PP
The behavior of the \fBpackage\fR command is determined by its first argument.
The following forms are permitted:
.TP
\fBpackage forget ?\fIpackage package ...\fR?
Removes all information about each specified package from this interpreter,
including information provided by both \fBpackage ifneeded\fR and
\fBpackage provide\fR.
.TP
\fBpackage ifneeded \fIpackage version\fR ?\fIscript\fR?
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 \fIscript\fR.
The script is saved in a database for use by subsequent
\fBpackage require\fR commands;  typically, \fIscript\fR
sets up auto-loading for the commands in the package (or calls
\fBload\fR and/or \fBsource\fR directly), then invokes
\fBpackage provide\fR 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 \fIpackage\fR
and \fIversion\fR, the new \fIscript\fR replaces the existing
one.
If the \fIscript\fR argument is omitted, the current script for
version \fIversion\fR of package \fIpackage\fR is returned,
or an empty string if no \fBpackage ifneeded\fR command has
been invoked for this \fIpackage\fR and \fIversion\fR.
.TP
\fBpackage names\fR
Returns a list of the names of all packages in the
interpreter for which a version has been provided (via
\fBpackage provide\fR) or for which a \fBpackage ifneeded\fR
script is available.
The order of elements in the list is arbitrary.
.TP
\fBpackage present\fR
This command is equivalent to \fBpackage require\fR except that it
does not try and load the package if it is not already loaded.
.TP
\fBpackage provide \fIpackage \fR?\fIversion\fR?
This command is invoked to indicate that version \fIversion\fR
of package \fIpackage\fR is now present in the interpreter.
It is typically invoked once as part of an \fBifneeded\fR script,
and again by the package itself when it is finally loaded.
An error occurs if a different version of \fIpackage\fR has been
provided by a previous \fBpackage provide\fR command.
If the \fIversion\fR argument is omitted, then the command
returns the version number that is currently provided, or an
empty string if no \fBpackage provide\fR command has been
invoked for \fIpackage\fR in this interpreter.  
.TP
\fBpackage require \fR\fIpackage \fR?\fIrequirement...\fR?
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.
.RS
.PP
A suitable version of the package is any version which satisfies at
least one of the requirements, per the rules of \fBpackage
vsatisfies\fR. 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 \fBpackage prefer\fR.
.PP
In the
.QW 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
.QW latest
selection mode the command will accept the highest version satisfying
all the requirements, regardless of its stableness.
.PP
If a version of \fIpackage\fR has already been provided (by invoking
the \fBpackage provide\fR command), then its version number must
satisfy the \fIrequirement\fRs and the command returns immediately.
Otherwise, the command searches the database of information provided by
previous \fBpackage ifneeded\fR 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 \fBpackage provide\fR for the package.
If the \fBpackage ifneeded\fR database does not contain an acceptable
version of the package and a \fBpackage unknown\fR 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 \fBpackage ifneeded\fR script for it.
If all of these steps fail to provide an acceptable version of the
package, then the command returns an error.
.RE
.TP
\fBpackage require \-exact \fIpackage version\fR
This form of the command is used when only the given \fIversion\fR
of \fIpackage\fR is acceptable to the caller.  This command is
equivalent to \fBpackage require \fIpackage version\fR-\fIversion\fR.
.TP
\fBpackage unknown \fR?\fIcommand\fR?
This command supplies a
.QW "last resort"
command to invoke during
\fBpackage require\fR if no suitable version of a package can be found
in the \fBpackage ifneeded\fR database.
If the \fIcommand\fR argument is supplied, it contains the first part
of a command;  when the command is invoked during a \fBpackage require\fR
command, Tcl appends one or more additional arguments giving the desired
package name and requirements.
For example, if \fIcommand\fR is \fBfoo bar\fR and later the command
\fBpackage require test 2.4\fR is invoked, then Tcl will execute
the command \fBfoo bar test 2.4\fR to load the package.
If no requirements are supplied to the \fBpackage require\fR command,
then only the name will be added to invoked command.
If the \fBpackage unknown\fR command is invoked without a \fIcommand\fR
argument, then the current \fBpackage unknown\fR script is returned,
or an empty string if there is none.
If \fIcommand\fR is specified as an empty string, then the current
\fBpackage unknown\fR script is removed, if there is one.
.TP
\fBpackage vcompare \fIversion1 version2\fR
Compares the two version numbers given by \fIversion1\fR and \fIversion2\fR.
Returns -1 if \fIversion1\fR is an earlier version than \fIversion2\fR,
0 if they are equal, and 1 if \fIversion1\fR is later than \fBversion2\fR.
.TP
\fBpackage versions \fIpackage\fR
Returns a list of all the version numbers of \fIpackage\fR
for which information has been provided by \fBpackage ifneeded\fR
commands.
.TP
\fBpackage vsatisfies \fIversion requirement...\fR
Returns 1 if the \fIversion\fR satisfies at least one of the given
requirements, and 0 otherwise. Each \fIrequirement\fR is allowed to
have any of the forms:
.RS
.TP
min
This form is called
.QW min-bounded .
.TP
min-
This form is called
.QW min-unbound .
.TP
min-max
This form is called
.QW bounded .
.RE
.RS
.PP
where
.QW min
and
.QW max
are valid version numbers. The legacy syntax is
a special case of the extended syntax, keeping backward
compatibility. Regarding satisfaction the rules are:
.RE
.RS
.IP [1]
The \fIversion\fR has to pass at least one of the listed
\fIrequirement\fRs to be satisfactory.
.IP [2]
A version satisfies a
.QW bounded
requirement when
.RS
.IP [a]
For \fImin\fR equal to the \fImax\fR if, and only if the \fIversion\fR
is equal to the \fImin\fR.
.IP [b]
Otherwise if, and only if the \fIversion\fR is greater than or equal
to the \fImin\fR, and less than the \fImax\fR, where both \fImin\fR
and \fImax\fR have been padded internally with
.QW a0 .
Note that while the comparison to \fImin\fR is inclusive, the
comparison to \fImax\fR is exclusive.
.RE
.IP [3]
A
.QW min-bounded
requirement is a
.QW bounded
requirement in disguise,
with the \fImax\fR part implicitly specified as the next higher major
version number of the \fImin\fR part. A version satisfies it per the
rules above.
.IP [4]
A \fIversion\fR satisfies a
.QW min-unbound
requirement if, and only if it is greater than or equal to the
\fImin\fR, where the \fImin\fR has been padded internally with
.QW a0 .
There is no constraint to a maximum.
.RE
.TP
\fBpackage prefer \fR?\fBlatest\fR|\fBstable\fR?
With no arguments, the commands returns either
.QW latest
or
.QW stable ,
whichever describes the current mode of selection logic used by
\fBpackage require\fR.
.RS
.PP
When passed the argument
.QW latest ,
it sets the selection logic mode to
.QW latest .
.PP
When passed the argument
.QW stable ,
if the mode is already
.QW stable ,
that value is kept.  If the mode is already
.QW latest ,
then the attempt to set it back to
.QW stable
is ineffective and the mode value remains
.QW latest .
.PP
When passed any other value as an argument, raise an invalid argument
error.
.PP
When an interpreter is created, its initial selection mode value is set to
.QW stable
unless the environment variable \fBTCL_PKG_PREFER_LATEST\fR
is set.  If that environment variable is defined (with any value) then
the initial (and permanent) selection mode value is set to
.QW latest .
.RE
.SH "VERSION NUMBERS"
.PP
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.
In addition, the letters
.QW a
(alpha) and/or
.QW b
(beta) may appear
exactly once to replace a dot for separation. These letters
semantically add a negative specifier into the version, where
.QW a
is \-2, and
.QW b
is \-1. Each may be specified only once, and
.QW a
or
.QW 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
.QW a
or
.QW b
as specified
above is called a \fBstable\fR version, whereas presence of the letters
causes the version to be called is \fBunstable\fR.
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.
.SH "PACKAGE INDICES"
.PP
The recommended way to use packages in Tcl is to invoke \fBpackage require\fR
and \fBpackage provide\fR commands in scripts, and use the procedure
\fBpkg_mkIndex\fR to create package index files.
Once you have done this, packages will be loaded automatically
in response to \fBpackage require\fR commands.
See the documentation for \fBpkg_mkIndex\fR for details.
.SH EXAMPLES
To state that a Tcl script requires the Tk and http packages, put this
at the top of the script:
.CS
\fBpackage require\fR Tk
\fBpackage require\fR http
.CE
.PP
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:
.CS
if {[catch {\fBpackage require\fR 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
}
.CE
.SH "SEE ALSO"
msgcat(n), packagens(n), pkgMkIndex(n)
.SH KEYWORDS
package, version