Multi-stage programs in context

Abstract

Cross-stage persistence is an essential aspect of multi-stage programming that allows a value defined in one stage to be available in another. However, difficulty arises when implicit information held in types, type classes and implicit parameters needs to be persisted. Without a careful treatment of such implicit information---which are pervasive in Haskell---subtle yet avoidable bugs lurk beneath the surface. This paper demonstrates that in multi-stage programming care must be taken when representing quoted terms so that important implicit information is kept in context and not discarded. The approach is formalised with a type-system, and an implementation in GHC is presented that fixes problems of the previous incarnation.