datatype number = Int  of int
                | Real of real

fun toReal (Int  i) = Real (real i)
  | toReal (Real r) = Real r

fun addNumbers (Int a) (Int b) = Int (a+b)
  | addNumbers nb1 nb2 =
       let
         val (Real r1) = toReal nb1
         val (Real r2) = toReal nb2
       in
         Real (r1+r2)
       end

val a = Int 10
val b = Real 15.0