Abstract
As popularity of algebraic effects and handlers increases, so does a demand for their efficient execution. Eff, an ML-like language with native support for handlers, has a subtyping-based effect system on which an effect-aware optimizing compiler could be built. Unfortunately, in our experience, implementing optimizations for Eff is overly error-prone because its core language is implicitly-typed, making code transformations very fragile. To remedy this, we present an explicitly-typed polymorphic core calculus for algebraic effect handlers with a subtyping-based type-and-effect system. It reifies appeals to subtyping in explicit casts with coercions that witness the subtyping proof, quickly exposing typing bugs in program transformations. Our typing-directed elaboration comes with a constraint-based inference algorithm that turns an implicitly-typed Eff-like language into our calculus. Moreover, all coercions and effect information can be erased in a straightforward way, demonstrating that coercions have no computational content.
Highlights
Algebraic effect handlers [17,18] are quickly maturing from a theoretical model to a practical language feature for user-defined computational effects
One must avoid the poisoning problem [26], where unification forces a pure computation to take the less precise impure type of the context. Since this rules out existing effect systems for handlers based on row-polymorphism [8,12,14], we propose a polymorphic explicitly-typed calculus based on subtyping
The main point of SkelEff is to show that we can erase the effects and subtyping from ExEff to obtain types that are compatible with a System F-like language
Summary
Algebraic effect handlers [17,18] are quickly maturing from a theoretical model to a practical language feature for user-defined computational effects. 4 presents ExEff, the core calculus, which combines explicit System F-style polymorphism with explicit coercions for subtyping in the style of Breazu-Tannen et al [3] This calculus comes with a type-and-effect system, a small-step operational semantics and a proof of type-safety. SkelEff is representative of Multicore Ocaml’s support for algebraic effects and handlers [6], which is a possible compilation target of Eff. By showing that the erasure from ExEff to SkelEff preserves semantics, we establish that ExEff’s coercions are computationally irrelevant and that, despite the existence of multiple proofs for the same subtyping, there is no coherence problem. The full version of this paper includes an appendix with omitted figures and can be found at http://www.cs.kuleuven.be/publicaties/rapporten/cw/CW711.abs. html
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