Software Design for Flexibility

Software Design for Flexibility

How to Avoid Programming Yourself into a Corner

By Chris Hanson and Gerald Jay Sussman

Strategies for building systems that can be adapted for new situations with only minor programming modifications.
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Strategies for building systems that can be adapted for new situations with only minor programming modifications.

Time pressures encourage programmers to write code that works well for a narrow purpose, with no room to grow. But the best systems are evolvable; they can be adapted for new situations by adding code, rather than changing the existing code. The authors describe techniques they have found effective—over their combined 100-plus years of programming experience—that will help programmers avoid programming themselves into corners.

The authors explore ways to enhance flexibility by:• Organizing systems using combinators to compose mix-and-match parts, ranging from small functions to whole arithmetics, with standardized interfaces• Augmenting data with independent annotation layers, such as units of measurement or provenance• Combining independent pieces of partial information using unification or propagation• Separating control structure from problem domain with domain models, rule systems and pattern matching, propagation, and dependency-directed backtracking• Extending the programming language, using dynamically extensible evaluators

Some of the techniques, such as dynamically extensible generic procedures, are extremely powerful but potentially dangerous. Each chapter includes exercises. Code is written in Scheme, a simple dialect of Lisp. Examples range from astronomy to automatic differentiation to an adventure game.


$55.00 X ISBN: 9780262045490 448 pp. | 6 in x 9 in


  • “Most systems need to succeed over time, not merely at a point in time. A fascinating exploration of predicative dynamic dispatch, metadata, and other techniques for building flexible systems that can be enhanced without breaking.”

    Rich Hickey

    author of Clojure and architect of Datomic

  • “Hanson and Sussman's Software Design for Flexibility has introduced additive programming, a game changer. An additive style allows for making changes to existing designs without the programmer's efforts looking like the work of a contortionist. With elegance, clarity, and care, they point out long-overlooked problems in software design and offer their Scheme-friendly, clever solutions. Enjoy!”

    Dan Friedman

    Professor of Computer Science, University of Indiana; author of The Little Prover