Adding Type Declarations to Interlisp.

The Interlisp programming system provides facilities which allow a programmer to provide type declarations for program variables, procedures and expressions. In some respects, these faciltiies resemble the type systems found in many contemporary high level languages. However, the nature of Lisp and Lisp programming requires them to be singificantly different from conventional type systems. Their most striking feature is a very powerful type description language which supports run-time, as well as static, type validation. In this paper, the design of the Interlisp type declaration facilities is described, illustrating how the characteristics of Lisp affected the design. Some suggestions are offered as to which aspects of these would provde useful in other contexts.