Reflex collection of counters

Main.hs

data AddRemove = Add Int | Remove Int deriving Show

currentId :: AddRemove -> Int
currentId = \case
  Add n    -> n
  Remove n -> n

toMap :: AddRemove -> M.Map Int (Maybe ())
toMap = \case
  Add n    -> M.singleton n (Just ())
  Remove n -> M.singleton (n + 1) Nothing

counter :: _ => m ()
counter = do
  down <- button "<"
  up   <- button ">"
  c <- foldDyn id (0 :: Int) $ leftmost
    [ (+1)         <$ up
    , (subtract 1) <$ down
    ]
  dynText (T.pack . show <$> c)

main = do
  add <- button "Add"
  rem <- button "Remove"
  listLen <- foldDyn (\case
                       Add n    -> \acc -> Add (currentId acc + n)
                       Remove n -> \acc -> Remove (currentId acc - n)
                     )
             (Add 0) $ leftmost
               [ Add 1    <$ add
               , Remove 1 <$ rem
               ]
  dynText (T.pack . show <$> listLen)
  el "div" $ void $
    listHoldWithKey mempty (toMap <$> updated listLen) (\_ _ -> counter)