Built on solid mathematical foundations, effect handlers offer a uniform and elegant approach to programming with user-defined computational effects. They subsume many widely used programming concepts and abstractions, such as actors, async/await, backtracking, coroutines, generators/iterators, and probabilistic programming. As such, they allow language implementers to target a single implementation of effect handlers, freeing language implementers from having to maintain separate ad hoc implementations of each of the features listed above.

Due to their wide applicability, effect handlers are enjoying growing interest in academia and industry. For instance, several effect handler oriented research languages are under active development (such as Eff, Frank, and Koka), as are effect handler libraries for mainstream languages (such as C and Java), effect handlers are seeing increasing use in probabilistic programming tools (such as Uber’s Pyro), and proposals are in the pipeline to include them natively in low-level languages (such as WebAssembly). Effect handlers are also a key part of Multicore OCaml, which incorporates an efficient implementation of them for uniformly expressing user-definable concurrency models in the language.

However, enabling effect handlers to scale requires tackling some hard problems, both in theory and in practice. Inspired by experience of developing, programming with, and reasoning about effect handlers in practice, we identify five key problem areas to be addressed at this Dagstuhl Seminar in order to enable effect handlers to scale: Safety, Modularity, Interoperability, Legibility, and Efficiency. In particular, we seek answers to the following questions:

• How can we enforce safe interaction between effect handler programs and external resources?
• How can we enable modular use of effect handlers for programming in the large?
• How can we support interoperable effect handler programs written in different languages?
• How can we write legible effect handler programs in a style accessible to mainstream programmers?
• How can we generate efficient code from effect handler programs?

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Danel Ahman, Amal Ahmed, Sam Lindley, and Andreas Rossberg