Staging with Class: A Specification for Typed Template Haskell

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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. In this talk, I will present my recent work which studies this unsound interaction and proposes a resolution, staged type class constraints, which is formalized in a source calculus that elaborates into an explicit core calculus. I will 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. The 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.

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