Why Don’t You Use Elm?

While achieving a small Angular app today, my thoughts stumbled across something that piques curiosity. There’re loads of frontend technologies running currently, and the trendiest one might be React. I’ve played rather reasonably with React as well as Backbone, and I am now diving seriously into Angular (you know, one has to make a living). 
Incidentally, I am a huge fan of functional reactive programming and Elm has definitely grasped all my love and devotion. Why aren’t more people learning Elm?

So, let me try to make a point. Each time one learns a new framework, one gets used to the intrinsic philosophy that this framework builds. 
React expects you to understand what components are and how they behave towards their properties, states and listeners. It also requires you to have an idea of what is happening at the rendering level with the virtual dom. 
Angular on the other hand, presents a slew of services you can inject in your controllers, factories, directives and how they play well with the two-way data-binding. I won’t carp about Backbone because you’ve got the idea.

Each framework demands some knowledge, a set of prerequisites that may echo from one to another but nonetheless stay disparate. Although they usually rely on strong theoretical concepts, they are often so deeply entangled in the framework that it is complicated to use them out of their initial context. You’ll get familiar with the React way, or the Angular way yet you’re not likely to re-use their concepts elsewhere on your own (mainly because you’ll be using another framework and be compliant with new patterns). 
Why don’t you learn Elm instead? Well I know, Elm is a different language (and a functional one, yuck!) and the learning curve is way too important.

Therefore, it seems familiar and comfortable, as the big majority of frontend frameworks is built on top of JavaScript, to stick with JavaScript as much as possible. Hence, it feels like there’s one less barrier to overcome; the underlying language is always the same, isn’t it? 
Having a quick look at the JavaScript in Angular, and the one in React, it really looks like two fairly different flavor of JavaScript. The former would rather use a Vanilla/ES5 JavaScript whereas the latter embraces ES6/ES7 with an extensive usage of destructuring and methods declaration shortcuts.
It sounds like two different — albeit from a common ancestor — languages. You end up learning two distinct languages. So what’s your excuse now?

Instead of learning a framework (and its underlying JavaScript flavor), you’re able to deal with a language which will by itself, intrinsically, offer you all the control and the behavior you’ll expect from a framework
Plus, because you are directly facing patterns and concerns which built your code, you’re getting stronger insights. There’s one remark though. I’ve been told the main issue with Elm was about reading the documentation. For non functional programming developers, it appears to be an impenetrable dialect whereas documentations from the trendiest frameworks are more “standard”, more usual. 
Inasmuch as it seems to be the only obstacle that keeps you away from Elm, let’s spend five more minutes together to break through this.

So here we are, talking about Hindley-Milner type signatures. Besides being
present in every functional programming language (like the mighty Haskell for instance), it is a powerful way of communicating insights. Trust me on that, once you can read them fluently, it becomes the best documentation you’ll ever experience
Let’s start with an excerpt from Elm’s documentation:

isEmpty: String -> Bool

First, there’s the function’s identifier isEmpty that names the signature. Then, we find two types separated by an arrow. Incidentally, this type system is seemingly relevant because Elm is a strongly statically typed language. Although it sounds rather obvious, it is worth a remark. 
So, the arrow basically means that given what is on the left side, your function will give you what is on the right side. In this case, this is straightforward, given a String, you’ll get a result of Bool type. 
What about several arrows?

repeat : Int -> String -> String

Well, the previous rule still applies. There’re now two different cases though. We either provide an Int to the repeat function, or we provide an Int and a String (in that order). In the second case, we get a String as a result.

repeat 2 "patate"
-- "patatepatate"

In the first case however, we obtain a function that will, given a String,
return a String. Assuming we provided 2 as an Int, then we could write the result the following way:

repeat2: String -> String
repeat2 =
repeat 2
repeat2 "patate"
-- "patatepatate"

Can you feel the expressiveness of the type system? Brace yourself, there’s more. Try to get this one by yourself:

length: List a -> Int

At first sight, this signature looks quite unusual. There’s a new type List but, with a parametric type-variable a. Should you haven’t guess it, this signifies that your List holds… something, something of type a. It could be an Int, it could be a Bool or it could be a List Int as well. It nonetheless ensures that your list is made only from elements of that same type. 
The eventual meaning of the signature is that given a List of something, it will always return an Int. By the by, this is fair enough as we don’t need to know what are the types of the elements of the list to know how many of them are in the list, right? 
One more before I let you go code some Elm.

map: (a -> b) -> List a -> List b

First of all, parentheses indicate here a coupling between variables; they enhance the readability and give us a tiny hint about the meaning of the whole function. They indicate the first parameter has to be a function, meaning that (a -> b) should be provided as a block.
Thus you’ll give a function that takes a type a and returns a type b. Moreover, we now have two different type-variables to identify two (possibly) different types. They don’t have to be different though, but they might be. It could be Int -> String, as well as Bool -> Bool. Yet, every a in the signature has the same type, and every b is consistent as well. 
Should you already be familiar with the map function in JavaScript, this is the Elmish version. Given a function from type a to type b and a List of a, you get a mapped List of b. For instance in JavaScript:

[“patate”, “autruche”].map(x => x.length)
// [6, 8]

And the equivalent in Elm:

List.map String.length [“patate”, “autruche”]
— [6, 8]

There are some more theory and syntax properties about Hindley-Milner signatures, however this is what is needed to cover almost all Elm signatures you’ll find in packages. Understanding these signatures gives you access to a whole new part of the documentation. In the same time, it even allows you to reason about your code and to build your application quite easily. 
So, what are you waiting for? There’re few dependencies, it’s blazingly fast, built for frontend developers, and it’s free from any framework shackles.
Elm really focuses on frontend developers needs in such a way that the gap between your experience and functional programming is much smaller. Lot of frameworks today seemingly borrow Elm core concepts on which they based their architecture (Cycle and React+Redux are good examples of that).

Now, go learn some Elm. And for the most curious, Evan Czaplicki about Elm.

Many thanks to Romain Pellerin, Nicolas Gaborit & Matthieu Pizenberg for the proof-reading and advice.