StartApp apps supply an update function of:

update : Action -> Model -> ( Model, Effects a)

As apps grow larger, and more nested, this complection of Model and Effects results becomes a real pain to manage. The wiring for calling a sub-component's update function, upacking the result pair, and re-packing into your own result pair, is very tedious.

Things become much cleaner if we split the effects out. First, we retype update as:

update : Action -> Model -> Model
update action model = ...

...then we supply a separate effects function of:

effects : Action -> ( Model, Model ) -> Effects Action
effects action (oldModel, newModel) = ...

(Often your effects function will just use newModel, but you may need access to the model before update was called.)

This separation of concerns cleans up each function, reduces wiring and makes testing a bit easier too.

To use it, split your functions as above, then in your StartApp.start call use:

import Exts.StartApp exposing (updateSeparately)
app : App Model
app =
  StartApp.start
    { ...
    , update = updateSeparately update effects
    , ...
    }