Retrofitting Effect Handlers onto OCaml

TL;DR

This paper presents an efficient implementation of effect handlers for OCaml, enabling modular programming with user-defined effects while maintaining compatibility and minimal performance overhead.

Contribution

The paper introduces the first retrofitting of effect handlers onto OCaml, addressing compatibility and performance challenges in a practical, efficient manner.

Findings

Imposes only 1% overhead on non-effect code

Maintains compatibility with stack inspection tools

Efficient for code utilizing effect handlers

Abstract

Effect handlers have been gathering momentum as a mechanism for modular programming with user-defined effects. Effect handlers allow for non-local control flow mechanisms such as generators, async/await, lightweight threads and coroutines to be composably expressed. We present a design and evaluate a full-fledged efficient implementation of effect handlers for OCaml, an industrial-strength multi-paradigm programming language. Our implementation strives to maintain the backwards compatibility and performance profile of existing OCaml code. Retrofitting effect handlers onto OCaml is challenging since OCaml does not currently have any non-local control flow mechanisms other than exceptions. Our implementation of effect handlers for OCaml: (i) imposes a mean 1% overhead on a comprehensive macro benchmark suite that does not use effect handlers; (ii) remains compatible with program analysis tools that inspect the stack; and (iii) is efficient for new code that makes use of effect handlers.