[saityi]

In Standard ML, it would be a type error to do that specifically:

    - if true then "abc" else 2.01;
    stdIn:1.2-1.30 Error: types of if branches do not agree [tycon mismatch]
      then branch: string
      else branch: real
      in expression:
        if true then "abc" else 2.01

You would need to define a new data type to wrap them:

    - datatype string_or_real = r of real | s of string;
    - if true then s "abc" else r 2.01;
    val it = s "abc" : string_or_real