An Optimised Flow for Futures: From Theory to Practice

Nicolas Chappe¹, Ludovic Henrio², Amaury Maillé³, Matthieu Moy⁴, and Hadrien Renaud⁵

The Art, Science, and Engineering of Programming, 2022, Vol. 6, Issue 1, Article 3

Submission date: 2020-09-30
Publication date: 2021-07-15
DOI: https://doi.org/10.22152/programming-journal.org/2022/6/3
Full text: PDF

Abstract

A future is an entity representing the result of an ongoing computation. A synchronisation with a “get” operation blocks the caller until the computation is over, to return the corresponding value. When a computation in charge of fulfilling a future delegates part of its processing to another task, mainstream languages return nested futures, and several “get” operations are needed to retrieve the computed value (we call such futures “control-flow futures”). Several approaches were proposed to tackle this issues: the “forward” construct, that allows the programmer to make delegation explicit and avoid nested futures, and “data-flow explicit futures” which natively collapse nested futures into plain futures.

This paper supports the claim that data-flow explicit futures form a powerful set of language primitives, on top of which other approaches can be built. We prove the equivalence, in the context of data-flow explicit futures, between the “forward” construct and classical “return” from functions. The proof relies on a branching bisimulation between a program using “forward” and its “return” counterpart. This result allows language designers to consider “forward” as an optimisation directive rather than as a language primitive.

Following the principles of the Godot system, we provide a library implementation of control-flow futures, based on data-flow explicit futures implemented in the compiler. This small library supports the claim that the implementation of classical futures based on data-flow ones is easier than the opposite. Our benchmarks show the viability of the approach from a performance point of view.