Elm’s syntax is similar to Haskell’s. I had tried to learn Haskell a long time ago but failed miserably because I couldn’t understand the types. I did not find Elm’s syntax to be a problem, and it was nice and simple, especially compared to Elixir, the last language I had learned. However, I think it was the first time in my life that I wished I had had a small syntax description or guide before diving too deep into the language. For me there were two confusing things:
Sometimes I saw a bunch of words that were separated by spaces or by commas, and it took me a bit of time to realise that function arguments are separated by spaces, but elements in a list or similar are separated by commas.
Compound types that have more than one word, like
List Int. That one is easy to figure out of course, but when I saw
Html MsgI had no idea what it meant. I’m still not completely sure, in fact.
The first point in particular has an implication: if you use the result of a function call as an argument for second function you must enclose the first function call in parentheses. This all seems super obvious in retrospect, but when staring at code that uses a DSL-like library to generate HTML in a language you’re starting to learn… well, it would have helped to have the first point spelled-out. Example:
ul  (List.map (tagView sectionPage) (ModelUtils.getSectionTags section))
Here we have a call to the function
ul that has two arguments: an empty list and another list, namely the result of the call to
List.map. Note how the whole
List.map must be enclosed in parentheses: otherwise, it would be interpreted as a call to
ul with four arguments, not two. In the same way, both arguments to
List.map must be enclosed in parentheses, too.
Although strictly speaking you don’t have to use it, the Elm Architecture is a fundamental part of Elm. It’s the high-level pattern for application architecture when writing Single-Page Applications in Elm, the main usecase for the language. The pattern is very similar to redux and friends in React, but it’s nicer and more natural in a functional, statically-typed language like Elm.
In short, it means separating your application in sub-applications that have a model, a React-style view produced from the model, a set of messages the view can send, and an update function that receives messages and changes the model. Besides, Elm supports commands and subscriptions, which gives a nice, clean interface for WebSockets and other things.
Although I’ve been looking forward to going back to ClojureScript, and in particular learn and play with Om Next, Elm is certainly a worthy contender and totally worth checking out, especially if you’re using React and you want to go one step further.
I admit I did get frustrated now and then with the static types, and at first a bit with the syntax (see the two points above) and the indentation. However, all in all I enjoyed learning and using Elm a lot, and it feels very clean, productive, and just nice to program in.
The application I wrote was very small, though, and I didn’t quite get to explore patterns in how to split an application in several sub-applications. I did read a bit about it but didn’t get to use anything fancy.