A generalized user interface for applications programs (II)

Reviewer: John L. Bennett

The programmer implementing a display manager intends to provide mechanisms by which a user (or an application-oriented person in contrast with a programming-oriented person) “can modify the user interface without having to tamper with the underlying application program.” The “Karlsruhe Screen-Based Application Support System” has been designed to meet this need. This system has been used with a character-oriented VT100 terminal attached to a computer running the VAX/VMS operating system. The Karlsruhe system allows use of a simple programming language interface to specify operations on the application data as it is formatted for display on the screen in accord with a display template. The system also uses the simple programming language interface to interpret the logic of user actions for sending data to the application. A full-screen editor is used to modify the display template when the user wants to change the geometry of the screen-layout. The approach of using logic and templates separate from the application avoids making the display system dependent upon any particular application programming language environment. The disk file describing the template is read in by the application program without requiring linking or compiling. The paper contains a detailed example showing: (1) steps taken to separate the screen management from the application program, (2) use of input data verification at the level of the screen management, (3) logic within a display, (4) a list of valid user commands, and (5) error messages. The design described by this practitioner (the author indicates a separate theoretical description is in preparation) will be of interest to other practitioners. It may also be of interest to those working on the theory of display managers and those who find it stimulating to consider worked examples. For instance, some kinds of scrolling are addressed, but I suspect the cases of fields partially displayed (because of horizontal scrolling) are not handled. Other interesting fundamental questions arise when one considers adding graphics to such a display manager. An important theoretical issue is how far one can go, and under what conditions, in successfully insulating the screen manager from application knowledge. On another dimension, the goal of the system design is to make the system “simple enough so that the end user controls the dialog” and, at the same time, “powerful enough to provide for user specification of screen geometry, input constraints, computation facilities, and display logic quite independently of the application system.” This is an empirical goal, and the author gives no indication of any testing that would show how well this goal has been met in practice.