Type-directed automatic incrementalization

Abstract

Application data often changes slowly or incrementally over time. Since incremental changes to input often result in only small changes in output, it is often feasible to respond to such changes asymptotically more efficiently than by re-running the whole computation. Traditionally, realizing such asymptotic efficiency improvements requires designing problem-specific algorithms known as dynamic or incremental algorithms, which are often significantly more complicated than conventional algorithms to design, analyze, implement, and use. A long-standing open problem is to develop techniques that automatically transform conventional programs so that they correctly and efficiently respond to incremental changes. In this paper, we describe a significant step towards solving the problem of automatic incrementalization: a programming language and a compiler that can, given a few type annotations describing what can change over time, compile a conventional program that assumes its data to be static (unchanging over time) to an incremental program. Based on recent advances in self-adjusting computation, including a theoretical proposal for translating purely functional programs to self-adjusting programs, we develop techniques for translating conventional Standard ML programs to self-adjusting programs. By extending the Standard ML language, we design a fully featured programming language with higher-order features, a module system, and a powerful type system, and implement a compiler for this language. The resulting programming language, LML, enables translating conventional programs decorated with simple type annotations into incremental programs that can respond to changes in their data correctly and efficiently. We evaluate the effectiveness of our approach by considering a range of benchmarks involving lists, vectors, and matrices, as well as a ray tracer. For these benchmarks, our compiler incrementalizes existing code with only trivial amounts of annotation. The resulting programs are often asymptotically more efficient, leading to orders of magnitude speedups in practice.