# Pastebin mWf23ECx
class Failing f where
  warn :: String -> a -> f a
  err :: String -> f a

newtype Coder f i o = Coder { runCoder :: i -> f o }
  deriving stock Functor
  deriving newtype (Semigroup, Monoid)
  deriving (Applicative, Monad) via (Star f i)

fromKey
  :: (Failing f, Show k, Ixed m, Index m ~ k, IxValue m ~ v)
  => k -> Coder f v o -> Coder f m o
fromKey k (Coder f)
  = Coder $ \m -> maybe (err $ "missing key: " <> show k) f $ m ^? ix k