Elm in Production: 25K Lines Later
If decoding json in Elm is considered hard, I'd recommend checking out miso (https://github.com/dmjio/miso), a Haskell re-implementation of the Elm arch. It has access to mature json libraries like aeson for that sort of thing, along with mature lens libraries for updating your model. Here's an example of decoding json with GHC.Generics using typeclasses. 
https://github.com/dmjio/miso/blob/master/examples/xhr/Main....