The ease of understanding, maintaining, and developing a large program depends crucially on how it is divided up into modules. The possible ways a program can be divided are constrained by the available modular programming facilities ("module system") of the programming language being used. Experience with the Standard-ML module system has shown the usefulness of functions mapping modules to modules and modules with module subcomponents. For example, functions over modules permit abstract data types (ADTs) to be parameterized by other ADTs, and submodules permit modules to be organized hierarchically. Module systems with such facilities are called higher-order, by analogy with higher-order functions. Previous higher-order module systems can be classified as either opaque or transparent. Opaque systems totally obscure information about the identity of type components of modules, often resulting in overly abstract types. This loss of type identities precludes most interesting uses of hi...

ages a programming language, the usual other slogans of modern software engineering apply. That is, the language must support the writing of programs that are reliable, easily modi#ed, e#cient, machine-independent, and formally describable. A request for proposals produced 15 preliminary language designs. The Defense Department chose four of these for further development. After a twoyear competition, it selected a winner. This language was christened #Ada" in honor of Ada Augusta, Countess of Lovelace, a co-worker of Babbage and the #rst programmer. Ada was created in the limelight. Many members of the academic and industrial computer science community contributed advice and criticism to the development process. The result is a language whose scope is ambitious. SIGPLAN Notices served as a forum for much of the debate surrounding the speci#cation and development process. Ada is at the far language end of the language-model spectrum.* The entire syntax