A generic algorithm for checking exhaustivity of pattern matching (short paper)

Abstract

Algebraic data types and pattern matching are key features of functional programming languages. Exhaustivity checking of pattern matching is a safety belt that defends against unmatched exceptions at runtime and boosts type safety. However, the presence of language features like inheritance, typecase, traits, GADTs, path-dependent types and union types makes the checking difficult and the algorithm complex. In this paper we propose a generic algorithm that decouples the checking algorithm from specific type theories. The decoupling makes the algorithm simple and enables easy customization for specific type systems.

Type-level programming with match types

“…The Scala compiler checks for pattern matching exhaustivity to prevent runtime exceptions caused by missing cases. Exhaustivity checking uses static knowledge about the class hierarchy (such as the sealed and final annotations) to check that every value in the scrutinee type is covered by the pattern clauses [Liu 2016]. Non-exhaustive patterns are compiled with an additional "catch-all" case which throws a runtime exception.…”