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

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.

Implementing a stepper using delimited continuations

A stepper is a tool that displays all the steps of a program's execution. To implement a stepper, we need to reconstruct each intermediate program from the current redex and the evaluation context. We regard evaluation contexts as delimited continuations and capture them using the control operators shift and reset. This enables us to implement a stepper concisely by writing an evaluator that is close to the standard big-step interpreter. Our implementation is a non-trivial application of shift and reset.

Delimited continuations for prolog

Delimited continuationsare a famous control primitive that originates in the functional programming world. It allows the programmer to suspend and capture the remaining part of a computation in order to resume it later. We put a new Prolog-compatible face on this primitive and specify its semantics by means of a meta-interpreter. Moreover, we establish the power of delimited continuations in Prolog with several example definitions of high-level language features. Finally, we show how to easily and effectively add delimited continuations support to the WAM.