How to Read *SICP* and *Software Design for Flexibility*

Introduction

Two of the most influential books in programming are Structure and Interpretation of Computer Programs (SICP) by Harold Abelson and Gerald Jay Sussman and Software Design for Flexibility: How to Avoid Programming Yourself Into a Corner by Chris Hanson and Gerald Jay Sussman. These books provide deep insights into how to think about programming, abstraction, and system design. This guide aims to help you approach these books effectively and explore their common themes.

Why Read These Books?

Both books emphasize the power of abstraction and the importance of designing software that remains flexible and adaptable. While SICP lays a strong foundation in computational thinking and the principles of programming, Software Design for Flexibility (SDF) builds on those principles to explore how to make software that is maintainable and extensible over time.

How to Read SICP

SICP is an introduction to computer science, but it is not a book for beginners in programming. It teaches programming as a means of understanding computation, using Scheme, a dialect of Lisp.

Reading Strategy:

1. Follow Along with the Code: Type out and run the examples in Scheme (or an equivalent Lisp dialect). MIT Scheme or Racket are good environments for this.
2. Do the Exercises: Many of the book’s insights come from working through the problems. The exercises are not just practice—they introduce new concepts that are essential to understanding later chapters.
3. Watch the Lectures: The MIT OpenCourseWare lectures based on SICP can help clarify complex topics.
4. Take Your Time: The book is dense and conceptual. Expect to spend months working through it, especially if you’re engaging deeply with the exercises.
5. Think in Terms of Abstractions: SICP introduces a layered approach to understanding programs, starting with simple procedures and moving toward complex abstractions like streams and interpreters.

Key Ideas from SICP:

• The power of abstraction in programming.
• The importance of building languages within languages.
• The relationship between functions, state, and time.
• The role of metalinguistic abstraction in structuring complex systems.

Example: Recursive Factorial in Clojure

(defn factorial [n]
  (if (<= n 1)
    1
    (* n (factorial (dec n)))))
 
(println (factorial 5)) ; 120

How to Read Software Design for Flexibility

SDF builds upon the ideas from SICP, shifting from learning programming fundamentals to designing systems that are flexible and adaptable. The book introduces practical techniques for writing software that doesn’t become rigid and difficult to modify.

Reading Strategy:

1. Review SICP Concepts: SDF assumes familiarity with SICP, so if you haven’t read SICP, it might be helpful to at least review its key ideas.
2. Understand the Motivation: Each chapter presents a principle followed by practical examples. Instead of rushing through the book, take the time to absorb why each technique matters.
3. Experiment with the Patterns: Try implementing the design techniques in a project of your own to see how they improve flexibility.
4. Compare with Your Own Code: Reflect on past software projects. Where did your code become inflexible? How would you apply the book’s ideas to improve those designs?
5. Look for Generalization: Many of the book’s techniques revolve around keeping code open-ended, avoiding premature commitment to rigid structures.

Key Ideas from Software Design for Flexibility:

• Avoiding early commitment in software design.
• Using dispatching, combinations, and reflective techniques to enable extensibility.
• Understanding how to delay decisions about representation and control flow.
• The idea that software should be structured like mathematical expressions, which remain manipulable and transformable.

Example: Using Higher-Order Functions for Flexibility in Clojure

(defn make-adder [x]
  (fn [y] (+ x y)))
 
(def add5 (make-adder 5))
(println (add5 10)) ; 15

What These Books Have in Common

1. Abstraction as a Core Principle

   • Both books emphasize abstraction as a tool for managing complexity and increasing code flexibility.
   • SICP introduces abstraction through functional decomposition, data abstraction, and metalinguistic abstraction.
   • SDF builds on this by showing how to design abstractions that remain adaptable over time.

2. Metalinguistic Techniques

   • SICP teaches the power of creating languages within languages (e.g., interpreters, macros, DSLs).
   • SDF applies these ideas to software design, demonstrating how to build systems that can be extended without modifying core logic.

3. Delaying Decisions for Greater Flexibility

   • Both books argue that committing to rigid structures too early leads to brittle systems.
   • SICP presents stream processing and lazy evaluation as ways to defer computation.
   • SDF introduces techniques like dynamic dispatch, combinators, and reflection to delay structural decisions in system design.

4. Understanding Programs as Evolving Entities

   • SICP teaches that a program is a structured expression of a process, evolving through layers of abstraction.
   • SDF extends this by focusing on how to write code that evolves gracefully, without breaking existing functionality.

5. Lisp as a Medium of Thought

   • Both books use Lisp (Scheme in SICP, Common Lisp in SDF) because of its powerful abstraction capabilities.
   • The books highlight how Lisp’s homoiconicity and expressiveness make it well-suited for metaprogramming and extensibility.

Example: Macros in Clojure

(defmacro unless [pred a b]
  `(if (not ~pred) ~a ~b))
 
(println (unless false "do this" "do that")) ; "do this"

Final Thoughts

Reading SICP and Software Design for Flexibility together provides a complete journey from understanding computation to designing maintainable and adaptable software systems. While SICP builds fundamental programming intuition, SDF refines those ideas into practical strategies for real-world software development.

If you engage deeply with both books—working through the exercises, experimenting with the concepts, and reflecting on how they apply to your own work—you will gain a profound understanding of software design that extends beyond mere coding skills.

Happy reading!