Higher-order modules and the phase distinction

In earlier work, we used a typed function calculus, XML, with dependent types to analyze several aspects of the Standard ML type system. In this paper, we introduce a refinement of XML with a clear compile-time/run-time phase distinction, and a direct compile-time type checking algorithm. The calculus uses a finer separation of types into universes than XML and enforces the phase distinction using a nonstandard equational theory for module and signature expressions. While unusual from a type-theoretic point of view, the nonstandard equational theory arises naturally from the well-known Grothendieck construction on an indexed category.