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Exppp - An EXPRESS Pretty Printer



Donald E. Libes


EXPRESS is a data modeling language. EXPRESS is relatively new having only been standardized in 1993. Today, few tools exist that automatically generate EXPRESS and correspondingly most EXPRESS is hand-written. In the future, we predict that all but a tiny fraction of EXPRESS will be computer generated or computer read. While perhaps only during debugging, much of it will be read by humans so it is important that it appear as easy to read as possible.

The Supplementary Directives for the Drafting and Presentation of ISO 10303 state rules and guidelines for layout of EXPRESS. These rules and guidelines are incomplete and inadequate for humans, no less for the purposes of writing software to perform such presentation. Instead they appear to be an attempt to codify how people write EXPRESS now, with an eye towards making this easy for humans.

We provide improvements on their rules. These new rules are aimed at what we believe will be the biggest producers of EXPRESS in the future - programs. We have encoded these rules into a software library that can be incorporated into other tools. It has been used successfully in tools produced at NIST.

NIST Interagency/Internal Report (NISTIR) - 5292
Report Number


Express, PDES, presentation, pretty print, STEP


Libes, D. (1993), Exppp - An EXPRESS Pretty Printer, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed April 14, 2024)
Created January 1, 1993, Updated November 10, 2018