Looking Out To Sea

Earlier today our little SICP study group agreed to start on Chapter 3 for our next meeting. There are five chapters in total so we are past the halfway mark and it’s been pretty interesting so far. To recap the recent stuff:

• Representing data structures as functions, and data as functions. Specifically, calculating with Church numerals and simulating cons, car and cdr without built-in operations.
• Manipulation of code as data, including symbolic differentiation of simple algebraic formulae. Converting “3x²” into “6x” with relevant simplification and grouping of symbols.
• Some straight-up data structure work — ordered lists, trees, sets — and using them to implement Huffman encoding.
• Organising multiple interacting systems, in this case a generic arithmetic package which works (almost) seamlessly on integers, rationals, reals, complex numbers and finally polynomial terms. Using this as a way of discussing functional and object-oriented design and introducing the expression problem.

I can definitely say that this last section, building a multi-module (for some definition of module) system made me really miss static types. Not being able to tell where functions were defined and called with different arities, or were using different symbols, until the problem manifested at runtime, was infuriating. The book is very interesting but the language is not to my taste. :-)

For comparison, here’s Yaron Minsky talking about using Ocaml for large scale programming:

Effective ML from Yaron Minsky on Vimeo.