title: Change a record value with a select in Elm
body: <p>I try to do a simple thing, but it's hard to me to understand how to code.
  Suppose this record</p>
  
  <pre><code>type alias Model =
    { month : Int
    , cost : Int
    , email : String
    }
  </code></pre>
  
  <p>I have 2 ways to work with this model:
  - either user changes month and it's dynamically change cost (cost = month * 10) in this example
  - or user submits and it send data to a server with a json format</p>
  
  <p>My view looks like:</p>
  
  <pre><code>durationOption duration =
    option [value (toString duration) ] [ text (toString duration)]
  
  view model =
    Html.div []
      [ 
      , input [ placeholder "my@email.com" ] []
      , select []
        (List.map durationOption [0..12]) -- month selector
      , Html.span [][text (toString model.total)] -- value automatically updated when users changes month value in the select
      , button [ onClick Submit ] [text "Send"]
      ] 
  </code></pre>
  
  <p>Unfortunately, i do not understand how to update values, how to just update the cost:</p>
  
  <pre><code>update : Msg -&gt; Model -&gt; (Model, Cmd Msg)
  update action model =
    case action of
      Submit -&gt;
        (model, Cmd.none)
      {-
      Calculate -&gt;
         ???? 
      -}
  </code></pre>
  
  <p>I think I have to call Calculate, but I really do not understand how to do. I ve read example from documentation but there is nothing with select ...
  Can someone help me please ? </p>