if 0 {[Richard Suchenwirth] 2003-06-27 - As yet another attempt to [teach programming to children], this tiny plaything (both in length of code and screen estate) does multiplication on the fly for the factors you enter into the two entries.

[http://mini.net/files/multiply.jpg]

All communication happens via textvariables and traces. I'm not sure whether this already counts as a minuscule '''Model-View-Controller''' example:
   * the variables a,b,c are the ''models''
   * the label .c is a ''view'' on the result, c
   * the entries .a, .b are ''controllers'' for the user to change the model
But in any case, it's a pretty minimal example '(also for fancy widget names - I didn't believe they could be called .* or .= before I tried ...)
}
 proc main {} {
    global a b c
    entry .a -width 5 -textvariable a
    label .* -text *
    entry .b -width 5 -textvariable b
    label .= -text =
    label .c -width 10 -textvariable c
    eval pack [winfo children .] -side left

    foreach factor {a b} {
	trace variable $factor w {recompute}
    }
 }
if 0 {In this proc that tries to recompute (that might fail if an entry has been cleared), we ignore the three arguments that a trace gets automatically - we know what we want:}
 proc recompute {- - -} {
    global a b c
    catch {set c [expr {$a * $b}]}
 }
 main

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[Arts and crafts of Tcl-Tk programming]