Viscid, a vi-like screen editor written in Lucid∗

In this paper we describe a full screen editor program written entirely in Lucid, a functional dataflow language. Our goal was to verify that pure nonprocedural languages can be used to program ‘realistic’ applications, including ones involving interactive I/O. The editor (which we call viscid) is similar to the vi editor of Berkeley UNIX(TM). Viscid is much simpler than vi but does handle basic features, such as inserting and deleting characters, opening and deleting lines, searching for patterns, and scrolling up and down. We give an overview of the viscid program together with a complete program for a simplified version. We discuss the techniques and methodology employed, and compare the Lucid program to a version written in Pascal. The Lucid program is about half the size of the Pascal version, more modular, and if anything easier to write. These advantages are due primarily to Lucid’s treatment of variables (time varying values) which has more in common with spreadsheet systems than with conventional imperative languages. We consider the remaining obstacles which must be overcome if nonprocedural languages are to be a practical alternative to C and Pascal.