'''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.

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See also:

   * [package]
   * [package ifneeded]
   * [package provide]
   * [package require]
   * [package unknown]

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