I decided to write a series about the experiences I had writing an Elm app during the last couple of months. I am trying to add a new post every one or two weeks.

The app I rewrote is is a BMI, body-index and body fat percentage calculator, that can be used to check if your weight is in a healthy range or track fitness progress. You can take take a look at:


I wrote the app for my self to track my personal results during certain kind of fitness trainings over weeks / months. That was more than 4 years ago. It was written in Ruby on Rails as a backend-centric app. Recently I wanted to add one or two new features and realized that most of my users use mobile devices and the page is not at all mobile friendly. So I thought it could be a fun training project, which also makes things nicer the people already using the app.

Client side App

This site is a great example of an app that can be done almost completely on the client side. Originally I was afraid of writing everything client side, but that was 2012 and testing and other libraries for Javascript where just not there yet.

Elm for the win

I decided to write the new version in Elm, using the great elm-mdl library for mobile support. I picked Elm because I like it for its type system, package system, error messages and tooling. … As well as the great community that I experienced as very kind and helpful.

I also thought about using Clojure + ClojureScript + Om, but even though I really like Clojure, I wanted to try a ML like type system on a bigger project. I could have also used Purescript, but my knowledge of advanced type systems is limited, therefore I thought Elm would be the easier language to start with.

2-3 months later

I am very happy that I was able to implement all the features that existed in the original version. It was a lot of fun and I enjoyed writing Elm code almost every day. However it was a lot of work. I underestimated the work that is necessary to learn language, a new eco system and write a “private production” app in it.

Whats next

Thanks for reading this intro. Next I will get more technical and report about my biggest learnings about Elm and related things … and there where a lot. I will cover “temporarily narrowing error types”, “fuzz testing”, and much more.