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Journal of Functional Programming Journal of Functional Programming

Article contents

Effekt: Capability-passing style for type- and effect-safe, extensible effect handlers in Scala

Part of: Effects and Handlers

Published online by Cambridge University Press:  31 March 2020

JONATHAN IMMANUEL BRACHTHÄUSER [Opens in a new window] ,
PHILIPP SCHUSTER  and
KLAUS OSTERMANN
JONATHAN IMMANUEL BRACHTHÄUSER
Affiliation:
University of Tübingen, Tübingen, Germany (e-mails: jonathan.brachthaeuser@uni-tuebingen.de, philipp.schuster@uni-tuebingen.de, klaus.ostermann@uni-tuebingen.de)
PHILIPP SCHUSTER
Affiliation:
University of Tübingen, Tübingen, Germany (e-mails: jonathan.brachthaeuser@uni-tuebingen.de, philipp.schuster@uni-tuebingen.de, klaus.ostermann@uni-tuebingen.de)
KLAUS OSTERMANN
Affiliation:
University of Tübingen, Tübingen, Germany (e-mails: jonathan.brachthaeuser@uni-tuebingen.de, philipp.schuster@uni-tuebingen.de, klaus.ostermann@uni-tuebingen.de)
Corresponding
E-mail address:
jonathan.brachthaeuser@uni-tuebingen.de
philipp.schuster@uni-tuebingen.de
klaus.ostermann@uni-tuebingen.de
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Abstract

Effect handlers are a promising way to structure effectful programs in a modular way. We present the Scala library Effekt, which is centered around capability passing and implemented in terms of a monad for multi-prompt delimited continuations. Effekt is the first library implementation of effect handlers that supports effect safety and effect polymorphism without resorting to type-level programming. We describe a novel way of achieving effect safety using intersection types and path-dependent types. The effect system of our library design fits well into the programming paradigm of capability passing and is inspired by the effect system of Zhang & Myers (2019, Proc. ACM Program. Lang.3(POPL), 5:1-5:29). Capabilities carry an abstract type member, which represents an individual effect type and reflects the use of the capability on the type level. We represent effect rows as the contravariant intersection of effect types. Handlers introduce capabilities and remove components of the intersection type. Reusing the existing type system of Scala, we get effect subtyping and effect polymorphism for free.

Type
Research Article
Information
Journal of Functional Programming , Volume 30 , 2020 , e8
Creative Commons
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
© The Author(s) 2020. Published by Cambridge University Press

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