There has been much work in recent years on extending ML with recursive modules. One of the most difficult problems in the development of such an extension is the double vision problem, which concerns the interaction of recursion and data abstraction. In previous work, I defined a type system called RTG, which solves the double vision problem at the level of a System-F-style core calculus. In this paper, I scale the ideas and techniques of RTG to the level of a recursive ML-style module calculus called RMC, thus establishing that no tradeoff between data abstraction and recursive modules is necessary. First, I describe RMC’s typing rules for recursive modules informally and discuss some of the design questions that arose in developing them. Then, I present the formal semantics of RMC, which is interesting in its own right. The formalization synthesizes aspects of both the Definition and the Harper-Stone interpretation of Standard ML, and includes a novel two-pass algorithm for recursive module typechecking in which the coherence of the two passes is emphasized by their representation in terms of the same set of inference rules.

We propose F � , a calculus of open existential types that is an extension of System F obtained by decomposing the introduction and elimination of existential types into more atomic constructs. Open existential types model modular type abstraction as done in module systems. The static semantics of F � adapts standard techniques to deal with linearity of typing contexts, its dynamic semantics is a small-step reduction semantics that performs extrusion of type abstraction as needed during reduction, and the two are related by subject reduction and progress lemmas. Applying the Curry-Howard isomorphism, F � can be also read back as a logic with the same expressive power as second-order logic but with more modular ways of assembling partial proofs. We also extend the core calculus to handle the double vision problem as well as type-level and termlevel recursion. The resulting language turns out to be a new formalization of (a minor variant of) Dreyer’s internal language for recursive and mixin modules.