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.
The package selection algorithms inside package require need to trust that the package ifneeded commands have provided accurate information. That is, in order for package selection to work properly, it needs to be true that the command
package ifneeded $pkg $version $script
accurately and truthfully indicates that
uplevel #0 $script
will cause the command
package provide $pkg $version
to be evaluated (or equivalent Tcl_PkgProvide call).
See also: