Freya: nullable types

Nullable types are special value types, created with the predefined System.Nullable[T] generic type, and a value type as its type argument, and having special support by the compiler. They were designed to support null values as in SQL.

Nullable types

The Nullable[T] class is declared like this:

Nullable = record[record T]
    // ...
end;

Two obvious consequences can be inferred from the above declaration:

System.Nullable is a value type, and so are any closed type obtained from this type.
The generic parameter from Nullable only accepts value types.

There's another constraint that cannot be inferred from the Nullable's declaration: though value types are the only acceptable types, you cannot pass another nullable type in the generic type parameter:

var
    n1: Nullable[Byte];           // Allowed.
    n2: Nullable[String];         // Error! String is a reference type.
    n3: Nullable[Nullable[Byte]]; // Error! Nullable cannot be nested.

Instead of explicitly using the Nullable type identifier, Freya allows you to declare a nullable type by appending a question mark to the base type reference:

var
    n1: Byte?;

Nullable types

Nullable types

See also