Type directed compilation of row-typed algebraic effects

Abstract

Algebraic effect handlers, introduced by Plotkin and Power in 2002, are recently gaining in popularity as a purely functional approach to modeling effects. In this article, we give a full overview of practical algebraic effects in the context of a compiled implementation in the Koka language. In particular, we show how algebraic effects generalize over common constructs like exception handling, state, iterators and async-await. We give an effective type inference algorithm based on extensible effect rows using scoped labels, and a direct operational semantics. Finally, we show an efficient compilation scheme to common runtime platforms (like JavaScript) using a type directed selective CPS translation.

Effect handlers for the masses

“…Existing implementations of libraries and languages for (algebraic) effect handlers are either translations to a high level language or involve a custom runtime implementation. High level implementations translate effect handlers into delimited continuations [Brachthäuser and Schuster 2017;Kammar et al 2013;Kiselyov and Sivaramakrishnan 2016], free monads [Kiselyov and Ishii 2015] or perform a source to source CPS translation into another high-level language Leijen 2017c]. Other implementations require a custom runtime that supports stack manipulation [Bauer and Pretnar 2015;Dolan et al 2017] or setjump / longjump [Leijen 2017a].…”

Versatile event correlation with algebraic effects

“…In addition, it features native support for algebraic effects, effect handlers and row-based effect polymorphism. λ cart is similar to Koka [Leijen 2017b], whereas the presentation borrows heavily from Biernacki et al [2018]. That is, we employ explicit type/effect abstraction and subtyping.…”

“…Similarly to managed exceptions, a command invocation com v shifts the control flow to the currently innermost handler with the capability to handle com. Just as Leijen [2017b], we express this by restricting the evaluation context X com , i.e., evaluation may focus under a handler application only if h does not handle com.…”

“…We further allow the usual let and letrec binding forms, sequencing e 1 ; e 2 as well as parameterized handlers [Kammar et al 2013;Plotkin and Pretnar 2009] in a notation similar to Koka, i.e., handler(x = e){h}, binding the result value of e to a variable x, which is accessible from all clauses of handler h. Due to space limitations, we elide the desugaring of parameterized handlers and refer to Leijen [2017b]. One may think of them as pure functions accepting the parameter value and yielding a first class handler.…”

“…We prototyped Cartesius in two languages with native support for algebratic effecs, Koka [Leijen 2017b] and multicore OCaml [Dolan et al 2017]. The implementations are available at:…”