Staging with class: A specification for typed template Haskell

Abstract

<jats:p>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.</jats:p> <jats:p> We study this unsound interaction and propose a resolution, staged type class constraints, which we formalize in a source calculus λ <jats:sup>⇒</jats:sup> that elaborates into an explicit core calculus <jats:italic>F</jats:italic> . 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. </jats:p> <jats:p>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.</jats:p>