Can be shortened to..

menu * {
  Calculator:calc 
}
alias calc { dialog -m calc calc }
dialog calc {
  title "Calculator"
  size -1 -1 116 62
  option dbu
  edit "", 1, 6 12 49 10, autohs
  edit "", 2, 66 12 49 10, autohs
  text "First number", 3, 7 3 48 8, center
  text "Second number", 4, 66 3 48 8, center
  box "Answer:", 5, 7 38 108 23
  button "+", 6, 7 24 27 12
  button "-", 7, 34 24 27 12
  button "*", 8, 61 24 27 12
  button "/", 9, 88 24 27 12
  edit "", 10, 9 46 102 10
}
on *:dialog:calc:sclick:*: {
  if ($did isnum 6-9) {
    var %sym = + - * /
    did -ra $dname 10 $iif($did(1) isnum && $did(2) isnum,$calc($did(1) $gettok(%sym,$calc($did - 5),32) $did(2)),NaN)
  }
}