With its advanced type system, the purely functional language Haskell is extremely well-suited for developing applications based on embedded domain specific languages (EDSL) for domains where the quality of software is critical to success (e.g., the financial sector). To the compiler an EDSL is no different from any other library written in Haskell; to the user of the EDSL, it is a library that allows her to express her solution to a problem in her domain. in a syntax (fluency) reminscent of that of the domain. But one problem continues to plague the end users of EDSLs: whenever a type error occurs, the encoding of the domain inside the host language Haskell leaks out in those error message. And this encoding is exactly what the EDSL developers were trying to hide! In this talk we demonstrate how type error diagnosis can be made domain-specific by type level programming. We have implemented this in the Glasgow Haskell Compiler, the industrial-strength compiler for Haskell. With relatively small modifications to the compiler, we can show huge gains in terms of domain-aware type error messages, and suggestions on how to fix a given type error. Our solution has more advantages: existing EDSLs can be easily wrapped with specialized error diagnosis, the wrapping is invisible to the end user, and the soundness of the type system is guaranteed by the compiler. The abstractions provided by type-level programming allow us to describe and reuse common patterns in type error diagnosis, lowering the entry barrier for DSL authors.
Dr. Jurriaan Hage is an assistant professor at Utrecht University. His work in programming technology is largely focused on two aspects: the optimisation of functional languages by means of type and effect systems, and type error diagnosis for strongly typed functional languages. He is currently the lead maintainer of the Helium compiler for learning Haskell.