Type Elaboration Example 3: Union Types

        datatype MyNum = IV of int | FV of real;

        val n = FV(1.5);
                val n = FV 1.5 : MyNum

This is really useful as a union type:

        fun Num_to_real (IV i) = real i
          | Num_to_real (FV f) = f ;

                val Num_to_real = fn : MyNum -> real

        fun Numsqrt n = FV(sqrt(Num_to_real n));

                val Numsqrt = fn : MyNum -> MyNum

        fun Numtrunc (IV i) = i
          | Numtrunc (FV f) = trunc f ;

                val Numtrunc = fn : MyNum -> int