Context-Dependent Type Error Diagnosis for Functional Languages

Customizable type error diagnosis has been proposed as a solution to achieve domain-specific type error diagnosis for embedded domain specific languages. A proven approach is to phrase type inferencing as a constraint-solving problem, so that we can manipulate the order in which constraints are solved, and associate domain-specific type error messages with specific constraints to be communicated to the programmer in case type checking fails. A major challenge in this area lies in scaling this idea up uniformly to a fully-featured (functional) language, that is, languages that go beyond the polymorphic λ-calculus. In this paper, we show how within the general framework Constraint Handling Rules we can achieve such uniformity and generality, while at the same time providing the necessary type error customizability in a natural way. A proof-of-concept implementation is provided for a Haskell-like language, including support for type classes, GADTs and higher-ranked types. However, our approach applies to any language for which a constraint-based non-backtracking formulation of the type system is available.