Functional programming has become a hot topic in the last few years for programmers, but non-programmers might not understand what we’re talking about. I can imagine them wondering, “Did programs not work before? Why are they suddenly ‘functional’ now?”

Earlier today, I was tweeting a bit about the challenge of explaining to family or primary school students what the big deal is all about. Even we programmers take some time to cotton on to the notion. I know I’d have to pause for a moment if someone asked me what Haskell offers over Python.

If you’re not a programmer and have been wondering what the big deal is, here’s my attempt to lay it out.

First, consider the time just before computers existed. By the 1920s, a kind of math problem called a decision problem led various people to learn how to investigate the problem solving process itself. To do this, we had to invent an idea we call computability, meaning to automate problem solving using small, reusable pieces. A couple of mathematicians tackled this idea in two different ways (though they each came to the same conclusion), and today we have two ways to think about computability as a result.

I’m partial to Alan Turing’s approach because it’s very practical. He envisioned a process that’s quite mechanical. In fact, we now call it a Turing machine, even though his machine never actually existed. It was more of a mental exercise than something he intended to build.

To solve a problem with a Turing machine, he would break a problem into a sequence of steps which would pass through the machine on an endless tape. The machine itself knew how to understand the steps on the tape, and it knew how to store information in its memory. As the steps on the tape passed through, one at a time, the machine would consult the tape and its own memory to figure out what to do. This usually meant modifying something in its memory, which in turn could affect the following step, over and over until the steps ran out. By choosing the right set of steps, when you were done, the machine’s memory would end up with the answer you needed.

Since that time, most computers and programs are based on this concept of stringing together instructions which modify values in memory to arrive at a result. Learning to program means learning a vast number of details, but much of it boils down to understanding how to break a problem into instructions to accomplish the same thing. Programs made this way would not be considered “functional.”

At the same time, another mathematician, Alonzo Church, came up with another approach called lambda calculus. At its heart, it has a lot in common with Turing’s approach: lambda calculus breaks up a problem into small parts called functions. Instead of modifying things in memory, though, the key proposition of a function is that it takes input and calculates a result—nothing more. To solve a problem this way, little functions are written to calculate small parts of the problem, which are in turn fed to other functions which do something else, and so on until you get an answer.

Lambda calculus takes a much more abstract approach, so it took longer to work out how to make programs with it. When we did, we called these programs “functional programs” because functions were so fundamental to how they worked.

Putting all this together, I think of functional programs as ones which do their jobs without stopping to take notes along the way. As a practical consequence, this implies a few odd things. The little niceties that come first nature to procedural programs—like storing values, printing out text, or doing more than one thing at once—don’t come easy to functional programs1. On the other hand, functional programs allow for understanding better what a program will do, since it will do the same thing every time if its input doesn’t change.

I think both approaches have something to offer, and in fact, most programs are made with a combination of these ideas. Turing proved neither approach was better than the other. They’re just two ways of ending up at the same result. Programmers each have to decide for themselves which approach suits best—and that decision problem can’t be solved by a program yet.