Model.purs

type ItemRef = Int

newtype Item = Item
  { text :: String
  , done :: Boolean
  , ref :: ItemRef
  }

derive instance newtypeItem :: Newtype Item _

newtype Model = Model
  { items :: M.Map ItemRef Item
  , selection :: Maybe ItemRef
  , nextRef :: ItemRef
  }

derive instance newtypeModel :: Newtype Model _

_items :: forall a . Traversal' Model Item
_items = _Newtype <<< (lens _.items (_ { items = _})) <<< traversed