I misunderstood the title at first. I thought this was a rewrite of the Elm compiler in Haskell. This confused me because I was pretty sure that Elm was already written in Haskell.

For anyone else who was confused like I was: this is more like a new frontend framework to implement the Elm architecture but in Haskell. Kind of the way Fable.io has a framework called Elmish, but you're writing in F#.