Reference to a sub \&name
Anonymous subroutine (closure) sub { ... }
A method name 'name'
Any of these can be provided with arguments by enclosing them and the
arguments in []. Here are some examples:
$mw->bind($class,"<Delete>" => 'Delete');
This will call $widget->Delete, the $widget being provided (by bind) as
the one where the Delete key was pressed.
While having bind provide a widget object for you is ideal in many cases it can be irritating in others. Using the list form this behaviour can be modified:
$a->bind("<Delete>",[$b => 'Delete']);
because the first element $b is an object bind
will call $b->Delete.
Note that method/object ordering only matters for bind callbacks,
the auto-quoting in perl5.001 makes the first of these a little more readable:
$w->configure(-yscrollcommand => [ set => $ysb]);
$w->configure(-yscrollcommand => [ $ysb => 'set' ]);
but both will call $ysb->set(args provided by Tk)
Another use of arguments allows you to write generalized methods which are easier to re-use:
$a->bind("<Next>",['Next','Page']);
$a->bind("<Down>",['Next','Line']);
This will call $a->Next('Page') or $a->Next('Line') respectively.
Note that the contents of the [] are evaluated by perl when the
callback is created. It is often desirable for the arguments provided
to the callback to depend on the details of the event which caused
it to be executed. To allow for this callbacks can be nested using the
Ev(...) "constructor".
Ev(...) inserts callback objects into the
argument list. When perl/Tk glue code is preparing the argument list for
the callback it is about to call it spots these special objects and
recursively applies the callback process to them.
sub {} closures. Such closures may be a more appropriate way of
achieving the same effect.