The Tk::Error method is invoked by perl/Tk when a background error occurs. Two possible implementations are provided in the distribution and individual applications or users can define a Tk::error method (e.g. as a perl sub) if they wish to handle background errors in some other manner.
A background error is one that occurs in a command that didn't originate with the application. For example, if an error occurs while executing a command specified with a bind of after command, then it is a background error. For a non-background error, the error can simply be returned up through nested Tcl command evaluations until it reaches the top-level code in the application; then the application can report the error in whatever way it wishes. When a background error occurs, the unwinding ends in the Tk library and there is no obvious way for Tk to report the error.
When Tk detects a background error, it saves information about the error and invokes the Tk::Error command later when Tk is idle.
Tk::Error is invoked by perl/Tk as if by the perl code:
$mainwindow->Tk::Error("error message",location ...);
$mainwindow is the MainWindow associated with widget which detected the error, "error message" is a string describing the error that has been detected, location is a list of one or more "locations" which describe the call sequence at the point the error was detected.
The locations are a typically a mixture of perl location reports giving script name and line number, and simple strings describing locations in core Tk or perl/Tk C code.
Tk will ignore any result returned by the Tk::Error command. If another error occurs within the Tk::Error command (for example if it calls die) then Tk reports this error itself by writing a message to stderr (this is to avoid infinite loops due to any bugs in Tk::Error).
If several background errors accumulate before Tk::Error is invoked to process them, Tk::Error will be invoked once for each error, in the order they occurred. However, if Tk::Error calls Tk->break, then any remaining errors are skipped without calling Tk::Error.
The Tk.pm file includes a default (AutoLoadable) Tk::Error that simply reports the error on stderr.
An alternate definition is provided via :
require Tk::ErrorDialog;
that posts a dialog box containing the error message and offers
the user a chance to see a stack trace showing where the
error occurred.