A category-theoretic account of program modules

The type-theoretic explanation of modules proposed to date (for programming languages like ML) is unsatisfactory, because it does not capture that the evaluation of type-expressions is independent from the evaluation of program expressions. We propose a new explanation based on ‘programming languages as indexed categories’ and illustrate how ML can be extended to support higher order modules, by developing a category-theoretic semantics for a calculus of modules with dependent types. The paper also outlines a methodology, which may lead to a modular approach in the study of programming languages.