Staging with class: a specification for typed template Haskell

Multi-stage programming using typed code quotation is an established technique for writing optimizing code generators with strong type-safety guarantees. Unfortunately, quotation in Haskell interacts poorly with type classes, making it difficult to write robust multi-stage programs.

We study this unsound interaction and propose a resolution, staged type class constraints, which we formalize in a source calculus 𝜆 ⇒ that elaborates into an explicit core calculus 𝐹 . We show type soundness of both calculi, establishing that well-typed, well-staged source programs always elaborate to well-typed, well-staged core programs, and prove beta and eta rules for code quotations.

Our design allows programmers to incorporate type classes into multi-stage programs with confidence. Although motivated by Haskell, it is also suitable as a foundation for other languages that support both overloading and quotation.