[gladimdim]

At work we developed internal tool in Elm + elm-mdl for listening to specific events and notifying developer on their status change. It takes about 5 minutes to understand what is going on in application by just reading Model/Update. One person can refactor almost whole app and be sure he did not break a thing. Most issues we got were in ports (JavaScript). Also the same page is running as Chrome Extension and Firefox Extension.

I can say that development performance was 3x than the same developers in JS. Plus you don't have to write unit tests and deal with grunt/gulp/npm packages nonsense.

We got only one runtime exception - our state was saved in localStorage and pushed into Elm.embed when page was opened. If this format changes in Elm/localStorage - Elm fails to start. It would be nice to get some promise rejected when you do Elm.embed() and it fails to initialize. But we can write our own Json.decoders and parse state inside application - in such case we can even write data migrators for user data.

Next stop for us will be to rewrite some views into Elm and integrate them into huge our JS project.

[omouse]

How many lines of code are you at? Have you written tests?

[gladimdim]

it is about 2000 lines

[jononor]

And how many people use the application per week?