The next part is the update function, which is equivalent to a reducer in Redux:
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
( model, Cmd.none )
This function is responsible for updating your program’s model in response to the messages it receives; the Elm Runtime calls it automatically when it receives a message. This function takes two arguments: the incoming message (which will be a value of the Msg type we defined above), and the current model. It should return an updated model and a command to run if you need to kick off a side effect such as an HTTP request (or Cmd.none if you don’t).
In this example, we just return the existing model without modification, and we perform no command. At this point, the only message we’ve said we can receive is a NoOp, so it makes sense that our update function that responds to these messages will effectively “do nothing”. Once again, this function is effectively a placeholder that we’ll expand as we add features to our program.