Relaxing Ownership with Immutability

Multiple ownership [4] introduced a novel approach to managing object ownership information called “owners-as-boxes”. Each object is placed in a “box” or context and the nesting relationship between “boxes” or contexts respects the object accesses that happen during the program’s execution. No restrictions are placed on which objects are allowed to access which other objects, rather the ownership contexts provide a hierarchical, as opposed to a more traditional, flat, view of the program’s memory. As with any ownership system, the objects never point ”into” another context, but can point outwards from the context they are in. We show how introducing object immutability, readonly references, and class immutability allows to safely change ownership contexts the objects belong to without breaking the ownership restrictions imposed by multiple ownership. We describe a type system for a language with ownership contexts and immutability, prove it sound, and state and prove both ownership and immutability guarantees. Thus, we show how immutability greatly benefits ownership-enabled languages.