Dysfunctional Programming

Wednesday 7 August 2013 at 15:05 BST

There's a problem with learning a new programming paradigm: you often have to learn a new language simultaneously. So at SoCraTes 2013 in Germany, I decided I'd run a couple of sessions designed to teach functional programming without having to learn Haskell (when I wasn't sitting out on the terrace explaining monads with a beer in one hand and a folding whiteboard on my lap). The first time, it was a simple two-hour workshop, and the second time, it was part of the code retreat, in which people applied the concepts to implementing the Game of Life.

In the past, I ran a workshop on functional programming in object-oriented languages. This time, I reworded it a little bit, and entitled the session "functional programming in your favourite language". In this workshop, I gave people an implementation of an immutable list and an explanation of how they work and set them to work determining whether a given poker hand contains a pair. Easy, right?

Well, no. There's one rule: no mutation. What does this mean?

1. No reassignment. Every named variable may only be assigned once. In Java and C#, you can enforce this by declaring all variables and fields as final or readonly.
2. No side effects. All behaviour, as well as all behaviour you invoke, must be completely pure. "Side effects" include I/O, random number generation and all other behaviour that depends on the software not running under a closed system. For the purposes of this exercise, assertions in your tests are OK, as we need some way to determine that the code does anything at all.
3. All expressions must result in a value. This means that you cannot call void methods or side-effecting control flow such as if, for or while in C-based languages. This is because they encourage reassignment. Expression-based branching constructs such as a ? b : c or b if a else c in Python are perfectly valid.
4. All functions must return a value.

All these rules apply not just to code you write, but also any code you invoke. The methods you call must be implemented according to all of the above rules.

Oh, and one more rule: you must not use a functional programming language (Haskell, Lisps or other more esoteric languages) or functional features (list transformations in Scala, Ruby or C#, for example). That's too easy.

Give it a try and let me know how it goes. This lot had fun.

Next time on Monospaced Monologues: everything I learnt at SoCraTes 2013.