Contexts

Typical update or view functions look like:

view : Model -> Html Msg
view model =
  ...

Or

update : Msg -> Model -> (Model, Cmd Msg)
update message model =
  ...

It is very easy to get stuck in thinking that you need to pass only the Model that belongs to this component. Sometimes you need extra information and is perfectly fine to ask for it. For example:

type alias Context =
  { model : Model
  , time : Time
  }

view : Context -> Html Msg
view context =
  ...

This function asks for the component model plus a time which is defined in its parent's model.