March 13 – 18 , 2016, Dagstuhl Seminar 16112

From Theory to Practice of Algebraic Effects and Handlers


Andrej Bauer (University of Ljubljana, SI)
Martin Hofmann (LMU München, DE)
Matija Pretnar (University of Ljubljana, SI)
Jeremy Yallop (University of Cambridge, GB)

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Dagstuhl Report, Volume 6, Issue 3 Dagstuhl Report
Aims & Scope
List of Participants
Dagstuhl's Impact: Documents available


Being no strangers to the Dagstuhl seminars we were delighted to get the opportunity to organize Seminar 16112. Our seminar was dedicated to algebraic effects and handlers, a research topic in programming languages which has received much attention in the past decade. There are strong theoretical and practical aspects of algebraic effects and handlers, so we invited people from both camps. It would have been easy to run the seminar as a series of disconnected talks that would take up most of people's schedules - we have all been to such seminars - and run the risk of disconnecting the camps as well. We decided to try a different format, and would like to share our experience in this executive summary.

On the first day we set out to identify topics of interest and organize working groups around them. This did not work, as everybody wanted to be in every group, or was at least worried they would miss something important by choosing the wrong group. Nevertheless, we did identify topics and within them ideas began to form. At first they were very general ideas on the level of major research projects, but soon enough people started asking specific questions that could be addressed at the seminar. Around those questions small groups began to form. Out of initial confusion came self-organization.

We had talks each day in the morning, with the schedule planned two days ahead, except for the first day which started by a tutorial on algebraic effects and handlers. We left the afternoons completely free for people to work in self-organized groups, which they did. The organizers subtly made sure that everybody had a group to talk to. In the evening, just before dinner, we had a "show & tell" session in which groups reported on their progress. These sessions were the most interesting part of the day, with everyone participating: some showing what they had done so far, and others offering new ideas. Some of the sessions were accompanied by improvised short lectures.

Work continued after dinner and late at night. One of the organizers was shocked to find, on his way to bed, that the walls of a small seminar room were completely filled with type theoretic formulas, from the floor to the ceiling. He was greatly relieved to hear that the type theory was not there to stay permanently as the Dagstuhl caretakers painted the walls with a special "whiteboard" paint. They should sell the paint by the bucket as a Dagstuhl souvenir.

We are extremely happy with the outcome of the seminar and the way we organized it. An open format that gives everyone ample time outside the seminar room was significantly boosted by the unique Dagstuhl environment free of worldly distractions. We encourage future organizers to boldly try new ways of organizing meetings. There will be confusion at first, but as long as the participants are encouraged and allowed to group themselves, they will do so. If a lesson is to be taken from our seminar, it is perhaps this: let people do what they want, but also make sure they report frequently on what they are doing, preferably when they are a bit hungry.

Summary text license
  Creative Commons BY 3.0 Unported license
  Andrej Bauer, Martin Hofmann, Matija Pretnar, and Jeremy Yallop

Related Dagstuhl Seminar


  • Programming Languages / Compiler


  • Algebraic effects
  • Handlers
  • Computational effects
  • Programming languages
  • Implementation techniques


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