This appendix summarizes the main things a reader needs to know about the programming environment we use. I hope it will provide a useful quick overview, but this appendix is no substitute for a textbook on functional programming like Cousineau and Mauny (1998) or Paulson (1991). There are numerous other texts on OCaml and CAML Light available online, e.g. a fairly comprehensive OCaml book and some old lecture notes on CAML Light by the present author.

Functional programming

OCaml supports several styles of programming, but its roots lie in functional programming, and almost all of our code is written in a purely functional style. In brief, the idea of functional programming is that a program is simply an expression, and execution means evaluation of the expression. Although this point of view may seem outlandish to those with experience of more traditional imperative programming, supported by common languages like C and Java, an expression-centric view is already familiar from other contexts such as spreadsheet programming.

The centrepiece of imperative programming is the successive modification, via assignment statements x = e or x := e, of a number of program variables, known collectively as the state. These assignment statements are invoked in a particular order using sequential execution (sometimes indicated by ‘;’) and built into more complex constructs using if tests, while loops and so on.

Functional programming represents a radical departure from this model.