The behavior of the package command is determined by its first argument. The following forms are permitted:
A suitable version of the package is any version which satisfies at least one of the requirements, per the rules of package vsatisfies. 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 package prefer.
In the “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 “latest” 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 satisfy the requirements 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.
where “min” and “max” are valid version numbers. The legacy syntax is a special case of the extended syntax, keeping backward compatibility. Regarding satisfaction the rules are:
When passed the argument “latest”, it sets the selection logic mode to “latest”.
When passed the argument “stable”, if the mode is already “stable”, that value is kept. If the mode is already “latest”, then the attempt to set it back to “stable” is ineffective and the mode value remains “latest”.
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 “stable” 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 “latest”.
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 }