Native Implementation of Mutable Value Semantics

Abstract

Unrestricted mutation of shared state is a source of many well-known problems. The predominant safe solutions are pure functional programming, which bans mutation outright, and flow sensitive type systems, which depend on sophisticated typing rules. Mutable value semantics is a third approach that bans sharing instead of mutation, thereby supporting part-wise in-place mutation and local reasoning, while maintaining a simple type system. In the purest form of mutable value semantics, references are second-class: they are only created implicitly, at function boundaries, and cannot be stored in variables or object fields. Hence, variables can never share mutable state. Because references are often regarded as an indispensable tool to write efficient programs, it is legitimate to wonder whether such a discipline can compete other approaches. As a basis for answering that question, we demonstrate how a language featuring mutable value semantics can be compiled to efficient native code. This approach relies on stack allocation for static garbage collection and leverages runtime knowledge to sidestep unnecessary copies.