NO.146 Programming and Reasoning with Algebraic Effects and Effect Handlers

Shonan Village Center

March 25 - 29, 2019 (Check-in: March 24, 2019 )


  • Oleg Kiselyov
    • Tohoku University, Japan
  • Sam Lindley
    • The University of Edinburgh, UK
  • Gordon Plotkin
    • The University of Edinburgh, UK
  • Nicolas Wu
    • University of Bristol, UK


Algebraic effects and effect handlers are becoming an increasingly popular approach for expressing and composing computational effects. There are implementations of algebraic effects and effect handlers in Clojure, F#, Haskell, Idris, Javascript, OCaml, PureScript, Racket, Scala, Scheme, Standard ML, and even C. There are full-fledged languages built around effects such as Eff, Frank, Links, Koka, and Multicore OCaml. Moreover, there is growing interest from industry in effect handlers. For instance, Facebook’s React library for JavaScript UI programming is directly inspired by effect handlers, and Uber’s recently released Pyro tool for probabilistic programming and deep learning relies on effect handlers. Such interest arises from the ease of combining in the same program independently developed components using algebraic effects and effect handlers.

The increased adoption and use of algebraic effects and effect handlers reveal and make pressing three main problems: reasoning, performance, and typing. These problems may appear disparate, but we believe there are in fact deep connections that bring them together.

Reasoning Algebraic effects are defined by a signature of operations and an equational theory that describes how the operations interact, providing direct support for reasoning. Effect handlers are modular interpreters for algebraic effects, giving meaning to such operations. Existing implementations of effect handlers dispense with equations, largely because many open questions remain about how to incorporate them into a programming language. A key question that this meeting will seek to address is how to reintroduce equations and other forms of reasoning back into the effect handlers picture. An important consideration here is how to combine equational theories for several interacting effects.

Performance The dominant implementation method, the free monad, is notably slower than the direct execution of side-effects where available. A range of approaches for improving performance are under active investigation. These include direct stack manipulation, in the case that continuations are used linearly, selective CPS translations, and fusion transformations. The jury is still out on which techniques work best in which situations.

Typing Programming in the large involves working with complex and interacting systems. Effect type systems are a powerful means of taming this complexity, in a way that is amenable for practical programming. Several different effect type systems have been introduced for algebraic effects and effect handlers. It is not clear yet precisely what the tradeoffs are between the different approaches. Many open questions remain over how best to support features such as generative effects, and how to leverage effect type systems to support reasoning and to improve performance.

  • Effect handlers for concurrent and distributed programming;
  • Effect handlers for generative effects (ML references, renaming effects, scoped effects, modularity, runST, existentials);
  • Effect handlers with behavioral types (parameterized monads, graded monads, type state, session types, answer type modification, dependent types);
  • Effect handlers and resource management;
  • Effect handlers for probabilistic programming.

To promote mutual understanding, we plan for the workshop to have substantial time available for discussion. Our hope is to emphasize tutorials, brainstorming, and working-group sessions, rather than mere conference-like presentations.

The field of effect handlers is thriving, and we believe that Shonan would be an ideal setting to bring researchers interested in the topic together. A previous meeting held at Dagstuhl in March 2016, entitled “From Theory to Practice of Algebraic Effects and Handlers” had more people willing to participate than it was possible to accommodate. Since then, the field has grown, and so we anticipate that there will be an abundance of interest in attending this meeting.