I don't like that we have to write our own onEnter function to make viewInput work.

By wrapping the input element in a form element we can achieve the same goal, write simpler code and keep the HTML semantic.

viewInput : String -> Html Msg
viewInput task =
  header
    [ class "header" ]
    [ h1 [] [ text "todos" ]
    , form
        [ onSubmit Add ]
        [ input
            [ class "new-todo"
            , placeholder "What needs to be done?"
            , autofocus True
            , value task
            , name "newTodo"
            , onInput UpdateField
            ]
            []
        ]
    ]

Now when you type a description and press ENTER a submit event is fired on the form. The onSubmit event handler detects the submit event with preventDefault and propagates the Add message to the update function.

What do you think? Is this version any better?