I spent this weekend diving back into Clojure or more specifically, Clojurescript. For those who don't know, Clojure is a Lisp that runs on the JVM. The solution it particularly seeks to "solve" is immutability. Clojurescript is Clojure that compiles to Javascript for web applications. Tooling aside, it's all pretty neat.

I didn't do anything earth shaking, just a quick implementation of Conway's game of life and Snake. While the code isn't particularly good, you can check them out here (Snake, Life).

The Life implementation got me thinking about the value of studying different languages or more specifically, different paradigms. A student who implements the game of life in Java will likely implement something very similar in C++ as both languages are Object Oriented / imperative. An implementation in Clojure or Haskell will likely look very different.

Let's look a little deeper.

Conway's Game of LIfe is a Cellular Automaton and when teaching a CS class in Java or C++ it's a really nice application of 2D arrays. On each turn, for each cell in the world, you calculate its living neighbors. If the cell is alive and it has 2 or 3 living neighbors, it stays alive to the next turn. If the cell isn't alive but has 3 living neighbors it will become alive. Otherwise the cell dies (or remains dead).

In a Java type language, given a 2D array this_turn with the current world and you might get something like:

It's a really nice application of 2D arrays and I've done it with my classes frequently over the years.

Another language I've done life in is NetLogo. In NetLogo, you have a world represented by an x,y grid with each cell in the grid known as a patch. You write a program and all the patches run it independently. A life solution might look like this:

ask patches [ 
  set n count nieghbors with [color =  red] ;; red cells will be alive
  ifelse color = red 
    [ ifelse n=2 or n = 3
        [set nextcolor red]
        [set nextcolor black]
    ] 
    [
      ifelse n=3
        [ set nextcolor red ]
        [ set nextcolor black ]  
    ]
ask patches [set color nextcolor]

My NetLogo is a little rusty so my syntax might be off but the idea is to:

1. have each patch count it's living neighbors and figure out it's next state - alive (red) or dead (black)
2. have each patch set its color to the new state

This is a very different solution. In NetLogo you never think about looping over the cells. It's much higher level. You think about each location as it's own entity whereas in the Java solution you're getting much more into the nitty gritty.

NetLogo and Java are very different languages and using them encourages you to solve problems in different ways. you'd likely never create a 2D array in NetLogo - you'd exploit the word as your array. Likewise you'd exploit the patches to distribute the work. In Java you'd probably wouldn't create a class to represent each patch in the array and then run each in a thread. That would be overkill for the problem at hand.

Well, what about Clojure?

Clojure isn't set up for 2D arrays, at least not as cleanly as a language like C++ or Java. One could use Java Interop - that is, the ability of Clojure to directly use Java but that would defeat the purpose of using Clojure. Clojure is all about lists, vectors, and maps. Instead of a 2D array of the world, I made a list (vector) where each element contained the location of a living cell. This is certainly more space efficient. It also let me operate on the world as a sequence of transformations to the data:

1. Calculate all the possible next cells (the cells in the list plus all their neighbors
2. For each possible next cell, count it's living neighbors and determine if the cell is alive in the next iteration
3. Replace the list of cells with these new cells.

(defn generate [state]
  "Generate and return the next state"
  (let [potential-cells (for [[x y] state [dx dy] cell-bloc-3x3]
                          [(+ x dx) (+ y dy)])
        next-state (for [cell potential-cells]
                     (if (alive? cell state) cell))]
    (into #{} (remove nil?(into #{} next-state)))))

You can see the complete code here.

This again is a very different way of thinking about the problem. In Java we looped over the world and made adjustments. Here we transformed the current set of cells into the next generations set of cells in a pipeline like fashion.

Another feature of Clojure that results from its focus on immutability is that in programs like these the computational part of the program and the presentation part (the graphics) are totally separate. We first transform the data and then the UI displays the current data. Clojure does this very nicely and maybe I'll write a post on it but for now, I just want to point out that this is very different from NetLogo. In fact, I completely finished the program for life before I even thought about the interface. In NetLogo the interface is an integral part of the language.

The point of all of this is that languages designed around different paradigms are worth exploring even if you never do real work in them. Even if you don't like or use Clojure you can take lessons from it's functional side and how it handles immutability and you can add those techniques to your toolkit when using C++ or Java. It doesn't mean you should write Java to look like Clojure but having a complete toolkit of paradigms to draw from and apply when appropriate can only be good for us and for our students.