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.

@article{paperbot1547,
  title = {Staging with class: a specification for typed template Haskell},
  author = {Ningning Xie and Matthew Pickering and Andres Löh and Nicolas Wu and Jeremy Yallop and Meng Wang},
  journal = {Proceedings of the ACM on Programming Languages},
  volume = {6},
  number = {POPL},
  year = {2022},
  doi = {10.1145/3498723}
}