On the expressive power of user-defined effects: Effect handlers, monadic reflection, delimited control

Forster, Yannick; Kammar, Ohad; Lindley, Sam; Pretnar, Matija

doi:10.1017/s0956796819000121

Cited by 33 publications

(36 citation statements)

References 61 publications

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“…Unlike exception handling, which discards the continuation of where an exception is raised, however, handlers can resume computation from the point at which the operation was called. The ability to resume a computation suspended by the operation call provides algebraic effects and handlers with the expressive power to implement control effects [Bauer and Pretnar 2015;Forster et al 2019;Leijen 2017]. In our language, we use the expression resume M to resume the computation of the handled expression with the value of M.…”

Section: Review: Algebraic Effects and Handlersmentioning

confidence: 99%

Signature restriction for polymorphic algebraic effects

Sekiyama

Tsukada

Igarashi³

2020

Proc. ACM Program. Lang.

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The naive combination of polymorphic effects and polymorphic type assignment has been well known to break type safety. Existing approaches to this problem are classified into two groups: one for restricting how effects are triggered and the other for restricting how they are implemented. This work explores a new approach to ensuring the safety of polymorphic effects in polymorphic type assignment. A novelty of our work lies in finding a restriction on effect interfaces. To formalize our idea, we employ algebraic effects and handlers, where an effect interface is given by a set of operations coupled with type signatures. We propose signature restriction, a new notion to restrict the type signatures of operations, and show that signature restriction is sufficient to ensure type safety of an effectful language equipped with unrestricted polymorphic type assignment. We also develop a type-and-effect system to enable the use of both operations that satisfy and do not satisfy the signature restriction in a single program.

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Section: Review: Algebraic Effects and Handlersmentioning

confidence: 99%

Signature restriction for polymorphic algebraic effects

Sekiyama

Tsukada

Igarashi³

2020

Proc. ACM Program. Lang.

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“…Schrijvers et al (2019) compare the expressiveness of effect handlers and monad transformers in the context of Haskell. Forster et al (2017Forster et al ( , 2019 compare effect handlers with monadic reflection and delimited control in a more abstract untyped setting; Piróg et al (2019) extend this approach in order to compare effect handlers with delimited control in the presence of types, making essential use of polymorphism.…”

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confidence: 99%

Doo bee doo bee doo

Convent¹,

Lindley²,

McBride³

et al. 2020

J. Funct. Prog.

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Abstract We explore the design and implementation of Frank, a strict functional programming language with a bidirectional effect type system designed from the ground up around a novel variant of Plotkin and Pretnar’s effect handler abstraction. Effect handlers provide an abstraction for modular effectful programming: a handler acts as an interpreter for a collection of commands whose interfaces are statically tracked by the type system. However, Frank eliminates the need for an additional effect handling construct by generalising the basic mechanism of functional abstraction itself. A function is but the special case of a Frank operator that interprets no commands. Moreover, Frank’s operators can be multihandlers which simultaneously interpret commands from several sources at once, without disturbing the direct style of functional programming with values. Effect typing in Frank employs a novel form of effect polymorphism which avoids mentioning effect variables in source code. This is achieved by propagating an ambient ability inwards, rather than accumulating unions of potential effects outwards. With the ambient ability describing the effects that are available at a certain point in the code, it can become necessary to reconfigure access to the ambient ability. A primary goal is to be able to encapsulate internal effects, eliminating a phenomenon we call effect pollution. Moreover, it is sometimes desirable to rewire the effect flow between effectful library components. We propose adaptors as a means for supporting both effect encapsulation and more general rewiring. Programming with effects and handlers is in its infancy. We contribute an exploration of future possibilities, particularly in combination with other forms of rich type systems.

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“…The connection between effect handlers and delimited control is not accidental. It has been established practically (Kiselyov & Sivaramakrishnan, 2018) as well as theoretically (Forster et al, 2017;Piróg et al, 2019) that certain forms of delimited continuations can express certain forms of algebraic effect handlers. Similarly, in the literature, effect handlers are sometimes introduced as a structured way to programming with delimited continuations (Kammar et al, 2013;Leijen, 2017c).…”

Section: Structured Programming With Delimited Continuationsmentioning

confidence: 99%

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

Brachthäuser¹,

Schuster²,

Ostermann³

2020

J. Funct. Prog.

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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.

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On the expressive power of user-defined effects: Effect handlers, monadic reflection, delimited control

Cited by 33 publications

References 61 publications

Signature restriction for polymorphic algebraic effects

Signature restriction for polymorphic algebraic effects

Doo bee doo bee doo

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

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