Haskell eXchange 2016

A few weeks ago I was able to attend the “Haskell eXchange” conference in London. In this post, I’d like to introduce readers to functional programming and briefly highlight the advantages and techniques of functional programming which I learned from a talk by Don Stewart, the author of the book “Real World Haskell”. Then I’ll share some thoughts about how companies might start to integrate functional programming into their tech stack.

Introduction to functional programming

What is functional programming?
A style of building programs using mathematical functions.

What is mathematical function?
A relation exists between a set of inputs and a set of outputs; each with the property that each input is related to exactly one output.

Let me give an example:

var x = 1;

function impure(y) {
  x = x + y;
  return x;
function pure(x) {
  return x + 1;

console.log(impure(3)); // 4
console.log(impure(3)); // 7

console.log(pure(3)); // 4
console.log(pure(3)); // 4

Impure functions return different result for the same input (number 3). In other words − impure functions have side-effects.

Why is this bad?
Because you lose modularity and it is harder to think about functions with side-effects.

On the other hand, it’s really easy to compose pure functions:

function compose(f,g) {
   return function(x) {
     return f(g(x));

function plusOne(x) {
  return x + 1;

var plusTwo = compose(plusOne, plusOne);
var plusThree = compose(plusTwo, plusOne);

console.log(plusOne(1)); // 2
console.log(plusTwo(1)); // 3
console.log(plusThree(1)); // 4

It’s like lego games; by having a set of pure functions you can easily build your own galaxy.

Why aren’t pure functions used all the time?
Pure functions are awesome but the real life programs have a bunch of side-effects. Getting a response from the server, reading from the file, printing to the screen − all these operations have side-effects. You can’t build a truly useful program just on top of pure functions − you need functions with side-effects as well.

Don Stewart’s talk

Haskell has a smart way to distinguish between pure and impure functions. It is common in the Haskell community to write function types for the functions.

makeApple :: Seed  IO Apple
makeJuice :: Apple  Juice

If you see IO (it’s a Monad) somewhere in type signature, it means the function is impure.
As well, Haskell has strong static typing. Before running a program, compiler should correctly type check a program (and compiler can catch a dozens of errors!).
Now, let me highlight some parts from Don Stewart’s talk. Don Stewart leads the Haskell teams in the financial sector. In his talk, he shared how to control complexity of applications with more than 3 million lines of code.

Here are a few tips from him:

• Types help to control complexity.

• Compare 2 pricing functions:

f :: Double  Double  String  Double
g :: Rate Libor  Spot SGD  Date  Rate SIBOR

g has more expressive types. In other words, you can say more about the function by looking in function type assuming you know financial domain.

• Remove unclear types.

• No side effects. Instead, try to write pure functions as many as possible.

• Make things simpler by controlling IO and new types.

• Types are for keeping code maintainable and self-documented.

• Use “new types” and “data” to distinguish unique entities in the system.

• Using Phantom types, you can tag the things. Make it impossible to mix up or combine values in nonsense ways.

• As opposed to Stings and Double types have too many valid values for most use cases, Bool often has too little information.
Instead of authenticate :: String -> String -> Bool
write authenticate :: Privileges p => User -> Password -> IO (AuthUser p)

• Lift errors into types (using Maybe and Either) for  making functions modular.

• Move partial functions to the edges and write total functions as a core of program.

• Types − in order to minimize complexity; it helps to deliver faster. Reuse is extremely cheap.

Of course, for the reader unfamiliar with Haskell, these tips don’t tell much, but let me repeat once more. Basically, the main idea is that you need to use meaningful types as much as possible. In such a way you give a compiler more information about a program and consequently, the compiler helps to catch a lot of errors and hopefully optimize the code. Other simple ideas are to use total functions, move side-effects to the edges of a program. That’s it!

Haskell to industry

Even if Haskell has a great community with a lot of academic folks behind it, the community is really small compare to the Python or Javascript, or Php communities. It tends to lack some useful libraries, so if you switch to Haskell you should expect to have to contribute a lot to existing libraries or write more libraries for your needs. However, I believe it should be changed soon since a lot of people have realized that the OOP paradigm is not an answer to robust software development, especially in the time when parallel and concurrent programming plays more important role. I understand that re-writing back-end systems on Haskell, if you were building the systems during last 10 years using Javasript, Php, Java may not be an option. As a first step towards Haskell I suggest a “Haskell-like” language − Elm. Why Elm? Elm looks a bit simpler to start and it’s web-browser-based, and so it looks like a good alternative to almighty Javascipt. One more advantage of Elm is that you can introduce it gradually into an existing JS project1. Once you are happy with your front-end using Elm you may think to move some of your back-end services to Haskell. After Elm it should be much easier to start.

Happy hacking!

At the end of this post I’d like to share a photo with one of the core Haskell developers, Simon Peyton Jones.

3min 51sec read

  1. How to use Elm at work

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