From an OO perspective, Elm lacks interfaces. Typeclasses are probably the best way to handle the idea of interfaces in the functional programming model.
Since functions are first class, you can bodge this with an object that has functions in it, but since those don't serialize they don't play well with a lot of Elm idioms if you put them in your model.
I thought about this, and kinda came to a conclusion, than I would take a language that has generics, and doesn't have interfaces (i.e. Elm) over a language that has interfaces and doesn't have generics (i.e. Go)
Since functions are first class, you can bodge this with an object that has functions in it, but since those don't serialize they don't play well with a lot of Elm idioms if you put them in your model.