I will be talking about Elm, an AltJS that has been the recent center of attention. Elm is a pure functional programming language. Usually, when we imagine a pure functional programming languages, they are often hard to work with, but Elm has some functions limited to have an easier learning curve. We have a profuct called Unipos, which is built with Elm with 6 front-end engineers. We have over 70k lines of Elm code in our production. This talk will be about my experience of using Elm in a large production, as well as some some trouble we had while making our product, Unipos.