Starting with version 3.0, callbacks are Scheme
closures. Apart scroll commands, callbacks are
Schemes procedures without parameter. Suppose for example, that we want to
associate a command with the previous .hello button. In
Tcl, such a command can be expressed as
In , we can write
When the user will press the mouse left button, the closure associated to the :command option will be evaluated in the global environment. Evaluation of the given closure will display the message and call the destroy Tk-command.