An object-command is a command created by calling an
text .t; # Creates text widget command .t
set i [image create photo]; # Creates photo image command
As the name implies, object-commands carry
along with them a data context.
A big problem with object commands is that the Tcl compiler
can not possibly validate them.
Such commands are usually assigned
to variables, and therefore code ends up with zillions
of calls like:
$w1 insert end "Start"
$o1 conf -state normal
Thus it is very difficult to tell (both for the compiler
and the maintainer) what the
above code is doing, and if it is correct.
Languages such as
C++ can manage this reliably because objects are
part of the
type system. Thus the compiler can warn developers
of improper usage. Tcl however has no types per-se
and thus the user is given no warning of a problem,
until the code is executed.
The lack of types (in the context
makes Tk applications very difficult to maintain.
Wize remedies this by
implementing alternative, non-object commands
for widgets, images etc.
Thus using these,
the above can be rewritten as:
Text insert $w1 end "Start"
Entry conf $o1 -state normal
photos blank $im
This makes it readily apparent (to later maintainers)
exactly what was intended by each command.
Moreover, when run with wize -Wall the Tcl compiler can
validate the following:
- Text has a sub-command insert with the given args.
- Entry has the sub-command conf with a -state option.
- photos has a blank sub-command.
© 2008 Peter MacDonald