Nullable type

A class type is not a super type or a subtype of null type. So, a variable of a class type may not be set to null. Only a variable of a nullable type may be set to null.

A nullable type is constructed from a class type. Suppose that T is a class type name. Then, T | null and T | undefined are nullable types (Note that null and undefiend are identical in BlueScript). A variable of T | null may be set to either a T object or null (or undefined).

let r: Rectangle | null = null
r = new Rectangle(3.0, 4.0)

A nullable type is also constructed from the string type and array types. All the following types are valid nullable types.

string | null
integer[] | null
string[] | null
Uint8Array | null

However, integer | null or float | null is not a valid type.

Subtyping

If type T is a subtype of S, then T | null is also a subtype of S | null.

Type guards

BlueScript (currently) supports only a very simple type guard. If a local varialbe of type T | null is tested for null in the condition expression of a if statement, that variable is treated as a variable of type T in the then clause or the else clause.

function area(r: Rectangle | null): float {
  if (r == null)
    return 0.0;
  else {
    // the static type of r is Rectangle.  Not Rectangle | null.
    return r.getArea();
  }
}

The expressions for testing must be either <variable> == null or <variable> != null.