Languages that support abstraction and modular structure, such as Standard ML, Modula, Ada, and (more or less) C++, may have deeply nested dependency hierarchies among source files. In ML the problem is particularly severe because ML’s powerful parameterized module (functor) facility entails dependencies among implementation modules, not just among interfaces.To efficiently compile individual modules in such languages, it is useful (in ML, necessary) to infer, digest, and cache the static environment resulting from the compilation of each module. Our system provides a simple model of compilation and linkage that supports incremental recompilation (a restricted form of separate compilation) with type-safe linkage. This model is made available to user programs in the form of a set of internal compiler modules, a feature that we call the “visible compiler”. The chief client of this interface is the IRM incremental recompilation manager from CMU.