## MLton 20241230

# GenerativeDatatype

In Standard ML, datatype declarations are said to be generative, because each time a datatype declaration is evaluated, it yields a new type. Thus, any attempt to mix the types will lead to a type error at compile-time. The following program, which does not type check, demonstrates this.

    functor F () =
       struct
          datatype t = T
       end
    structure S1 = F ()
    structure S2 = F ()
    val _: S1.t -> S2.t = fn x => x

Generativity also means that two different datatype declarations define different types, even if they define identical constructors. The following program does not type check due to this.

    datatype t = A | B
    val a1 = A
    datatype t = A | B
    val a2 = A
    val _ = if true then a1 else a2

## Also see

- GenerativeException