basics.elm

type Response
  = ...

type Action
  = Updated (Maybe Response)

decoder : Decoder Response  -- implementation elided
url : String                -- ditto

update : Action -> Model -> ( Model, Effects Action )
update action model =
  case action of
    Updated maybeResponse ->
      case maybeResponse of
        Just response ->  -- do things
        Nothing ->        -- /shrug

requestUpdate : Effects Action
requestUpdate =
  Http.get decoder url   -- Task Error Response
    |> Task.toMaybe      -- Task never (Maybe Response)
    |> Task.map Updated  -- Task never Action
    |> Effects.task      -- Effects Action