Sep 22, 2016: 9:30am - 1:00pm

T2: Extensible Effects: understanding them, implementing them, using them

Oleg Kiselyov


Effects is a model of side-effects as an interaction between a program and a handler: an authority in charge of resources that receives and acts on program requests. Effects let us represent side-effects in pure computations. The explicitness about possible and not possible side-effects, the localized handling, controlled interactions and encapsulation, the ease of reasoning make effects useful even in impure languages, which can do side-effects natively.

There are several implementations of effects in Haskell and Scala. Effects are natively supported in Idris and soon to be in OCaml. They are the defining feature of PureScript. Finally, the language Eff is built on algebraic effects and local effect handling. By nature effects are extensible, letting us easily combine in the same program independently developed components, each with their own effects. In the narrow sense, extensible effects refer to a specific implementation method, often used in Haskell, Scala, and even in Coq.

This tutorial aims first to explain the ideas behind the extensible effects, on simple to understand toy implementations. Then, using one real, optimized implementation, we will experience defining and handling various effects, on progressively more complex examples: catching IO exceptions in the presence of other effects, custom interactive parsing, non-determinism with committed choice (LogicT), semi-automatic management of file handles and other resources through monadic regions.

I intend the tutorial to be interactive, working through problems with active audience participation.

Tutorial objectives

By the end of the tutorial, the participants will hopefully:

  • Think of effects not as lifting State through two Error and Reader but as a query or a transformation on resources such as memory, configuration data, schedulers, oracles, etc. Written formally, this transformation often takes the form of an effect handler;
  • Define their own effects and handlers, as befits the task at hand;
  • Learn to control the interactions between effects;
  • Have the background for their own implementation of effects.

Programming language: Haskell, although I will strive (especially in the first, understanding part) to use a very simple subset of Haskell that could be easily mapped to other functional languages such as Scala or OCaml.

Target audience

Users, researchers, students of functional programming. Basic knowledge of Haskell, ML or other typed higher-order functional language is assumed.

Infrastructure Required

I intend to work through many exercises, with active audience participation. It will be very helpful for the participants to have a laptop with the GHC installed (and being able to download tutorial materials, before or during the tutorial). No libraries and packages beyond the standard library will be needed. The most basic installation should suffice.

Oleg Kiselyov

Oleg Kiselyov

Oleg Kiselyov is an Assistant Professor at the Tohoku University, Japan. More information is on the web site