#!/usr/bin/wish

entry .n1 -width 3
label .add -text " + "
entry .n2 -width 3
button .eq -text " = " -command do_it
label .result -width 4

pack .n1 .add .n2 .eq .result -side left -fill both

# =================================================================

# an event handler
proc do_it {} {
    set sum [expr [.n1 get] + [.n2 get]]
    .result configure -text $sum
}