Union

Unions allow a value to potentially have multiple types.

union Number {
  int,
  decimal, // trailing comma allowed
}

Note that the Number type defined by the union is not directly compatible with any of its constituent types, and is a separate new type of its own. So for instance, you cannot add a Number to an int, even if your Number value is an integer, at least not without defining your own fn add(Number a, int b) function.

In order to get the constituent type information out of the union, you can do a match on the type like so:

Number a = 3.14 // The type is Number, but it's a decimal inside

match typeof(a) { // typeof() is a builtin function
  int -> {
    // `a` has a type of int here, you can use it as such
    print("{a} is an int")
    print(a + 5) // valid
  }

  decimal -> { print("{a} is a decimal") }
}

Notice that there's no need for an else case because that's all the possibilities for the type.