Chapter 2 Distilled

Outline of the Chapter

This chapter in SICP introduces cons cells as a way of combining data objects to form compound data, uses them to build lots of different data structures and talks about data abstraction

We stick with the books presentation at first, working through the rational number example with cons, car and cdr.

Then we learn about Clojure's datatypes and reimplement rationals using them. We mostly use them from then on (sometimes summarising the books approach in contrast).

What changed

The book builds up data structures from cons cells it uses them throughout the book. It is good in the sense you have a minimal set of primitives and build everything from them. However, if you have used another programming language you probably expect lots of datastructures to be built in. Clojure is particularly nice here compared to the Scheme in the book, having lots of data structures, a literal syntax for most of them, and immutability.

I think having to roll-your-own datastructures clouds the description of more complex topics like the interpreter, the presentation here gains a lot from relying on Clojure maps to represent environments for example.

Was was left out

I find the extended exercise on interval arithmetic boring so left it out. Despite enjoying it myself and it prefiguring the interpreter in Ch4 somewhat I have (at least for now) left out the Symbolic Differentiation example as I think the 'one has to learn calculus to do SICP' meme is not helpful.

What was added

I make a bit more of a fuss about Church Numerals as I think it is a wonderful a-ha moment potentially buried in the questions.

Obviously talking about Clojure's built-in data types is different and I think it clarifies the presentation of sets and coding.