Reference Capabilities for Trait Based Reuse and Concurrency Control

The proliferation of shared mutable state in object-orientedprogramming complicates software development as two seeminglyunrelated operations may interact via an alias and produceunexpected results. In concurrent programming this manifestsitself as data-races. Concurrent object-oriented programming further suffers from thefact that code that warrants synchronisation cannot easily bedistinguished from code that does not. The burden is placed solelyon the programmer to reason about alias freedom, sharing acrossthreads and side-effects to deduce where and when to applyconcurrency control, without inadvertently blocking parallelism. This paper presents a reference capability approach to concurrentand parallel object-oriented programming where all uses of aliasesare guaranteed to be data-race free. The static type of an aliasdescribes its possible sharing without using explicit ownership oreffect annotations. Type information can express non-interferingdeterministic parallelism without dynamic concurrency control,thread-locality, lock-based schemes, and guarded-by relationsgiving multi-object atomicity to nested data structures.Unification of capabilities and traits allows trait-based reuseacross multiple concurrency scenarios with minimal codeduplication. The resulting system brings together features from awide range of prior work in a unified way.Structured AliasingUpscaleUPMAR