Multiple ownership

Abstract

Existing ownership type systems require objects to have precisely one primary owner, organizing the heap into an ownership tree. Unfortunately, a tree structure is too restrictive for many programs, and prevents many common design patterns where multiple objects interact. Multiple Ownership is an ownership type system where objects can have more than one owner, and the resulting ownership structure forms a DAG. We give a straightforward model for multiple ownership, focusing in particular on how multiple ownership can support a powerful effects system that determines when two computations interfere-in spite of the DAG structure. We present a core programming language MOJO, Multiple ownership for Java-like Objects, including a type and effects system, and soundness proof. In comparison to other systems, MOJO imposes absolutely no restrictions on pointers, modifications or programs' structure, but in spite of this, MOJO's effects can be used to reason about or describe programs' behaviour.

Ownership Types for Object Synchronisation

“…For simplicity we do not adopt these earlier schemes in this paper. In addition, like [5], our type system does not distinguish read and write effects as done in [8,6]. This is somewhat over-conservative in that it does not allow simultaneous reads of an object.…”

“…Effect annotations enhance program reasoning [6,4]. Our type system uses explicit and checkable type and effect annotations as a means of recording programmer intent.…”

“…Here we use ownership types to describe effects as structured sets of objects, similar to what has been done by others [5,8,23,6,10], without enforcing encapsulation. First we provide a quick overview of ownership types, and indicate how our model of structured objects allows us to statically reason about objects being disjoint, guaranteeing non-aliasing properties for program variables.…”

“…The label on the transition ε is the effect of the transition, which may either be empty (∅) or a singleton location ({l}). For simplicity, we adopt the object creation semantics from [6] where all fields are initialised with new objects, hence [R-NEW] has no side effect.…”

“…Ownership types [8,9] allow programmers to structure the object store hierarchically as an ownership tree; ownership-based effects [5,6,8] use ownership tree to reason about side effects in object-oriented programs and to capture potential conflict between tasks. A major contribution of the paper is the ability to automatically infer finer-grained synchronisation requirements for parallel tasks, sufficient to prevent any potential interference between them.…”