A strong module system is a very important language tool for
developing software systems. Classes alone do not allow for sufficient
levels of abstraction and separate compilation. Modules can be very
helpful in organizing code, providing abstraction, and supporting
separate compilation. Abstraction makes it difficult to share types
between modules, but transparent types can propagate too much
information to allow separate compilation. The use of partially
abstract types and manifest types can help to avoid these problems.

Earlier work by Robert van Gent and Angela Schuett under the direction
of Professor Kim Bruce resulted in the design and implementation of
the language PolyTOIL, a type safe object-oriented language with
strong polymorphic features. LOOM is a direct descendant of PolyTOIL
which omits subtyping in favor of a more flexible version of matching,
including matching-based subsumption. We give an overview of LOOM and
of a prototype interpreter for the language. Proofs of the complexity
of the matching algorithm and the decidability of type checking are
presented. We describe the design and implementation of a module
system for LOOM, and present an in-depth discussion of the issues that
motivated and affected the design process. Formal type checking and
semantic rules are given, and the prototype implementation is
described. The module system is evaluated, and proposals are made for
further work.