This paper is an initial report on a system, called PegaSys, that supports a new visual paradigm for describing and manipulating programs and their designs. A program design is described by a hierarchically structured collection of graphical pictures. These pictures are intended to be easy to understand, yet still serve as formal program documentation. Pictures in PegaSys capture a variety of concepts, including (but not limited to) what we believe to be the important design concepts represented in flowcharts, structure charts, dataflow diagrams, and module interconnection languages. PegaSys treats pictures as computationally meaningful structures, checking whether they make sense syntactically, whether hierarchical picture refinements adhere to certain methodological principles, and whether a picture is logically consistent with a program. This paper describes the key ideas that enable PegaSys to provide these capabilities, as well as important properties of the PegaSys design and underlying logical framework. PegaSys has been implemented on Xerox personal computers and takes advantage of a high-resolution bitmap display.
PegsSys is an experimental system in which a user formally describes how a program is put together by means of a hierarchically structured collection of pictures, called formal dependency d+tmae (FDDs). Icons in an FDD denote a wide range of data and control dependencies among the relatively coarsegrained entities contained in large programs. Dependencies considered atomic with respect to one level in a hierarchy can be decomposed into a number of dependencies at a lower level. Each dependency can be a predefined primitive of the FDD language or it can be defined by a PegaSys user in terms of the primitives.A PegsSys user is given the illusion that logical formulas do not exist, even though PegaSys reasons about them internally. This involves (1) checking whether an FDD is meaningful syntactically, (2) determining whether hierarchical refinements of an FDD are methodologically sound, and (3) deciding whether an FDD hierarchy is logically consistent with the program that it is intended to describe. The techniques used to provide these capabilities are discussed along with the logical properties that enable PegaSys to maintain the user illusion.
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