Introducing Institutions

There is a population explosion among the logical systems being used in computer science. Examples include first order logic (with and without equality), equational logic, Horn clause logic, second order logic, higher order logic, infinitary logic, dynamic logic, process logic, temporal logic, and modal logic; moreover ~, there is a tendency for each theorem prover to have its own idiosyncratic logical system. Yet it is usual to give many of the same results and applications for each logical system; of course, this is natural in so far as there are basic results in computer science that are independent of the logical system in which they happen to be expressed. But we should not have to do the same things over and over again; instead, we should generalize, and do the essential things once and for all! Also, we should ask what are the relationships among all these different logical systems. This paper shows how some parts of computer science can be done in any suitable logical system, by introducing the notion of an ins t i tu t ion as a precise generalization of the informal notion of a mlogical system, m A first main result shows that if an institution is such that interface declarations expressed in it can be glued together, then theories {which are just sets of sentences) in that institution can also be glued together. A second main result gives conditions under which a theorem prover for one institution can be validly used on theories from another; this uses the notion of an institution morphism. A third main result shows that institutions admiting free models can be extended to institutions whose theories may include, in addition to the original sentences, various kinds of constraints upon interpretations; such constraints are useful for defining abstract data types, and include so-called • data," "hierarchy, • and regenerating" constraints. Further results show how to define insitutions that mix sentences from one institution with constraints from another, and even mix sentences and {various kinds of) constraints from several different institutions. It is noted that general results about institutions apply to such mmultiplex" institutions, including the result mentioned above about gluing together theories. Finally, this paper discusses some applications of these results to specification languages, showing that much of that subject is in fact independent of the institution used.