todolist-update-with-modify.elm

update : Action -> Model -> Model
update action model =
  case action of
    Add ->
      let newTodo = ( model.nextID, Todo.init model.nextID model.addText )
          newTodos = model.todos ++ [ newTodo ]
      in
          { model |
              todos = newTodos,
              nextID = model.nextID + 1,
              addText = ""
          }
    InputText str ->
      { model |
          addText = str
      }
    Modify id todoAction ->
      let updateTodo (todoId, todoModel) =
        if todoId == id
          then (todoId, Todo.update todoAction todoModel)
          else (todoId, todoModel)
      in
        { model |
            todos = List.map updateTodo model.todos
        }