gen_server.hs

instance GenServerState Command Int CounterState where
  handle_call Get = gets counter
  
  handle_cast Inc =
    modify $ \st -> st { counter = counter st + 1 }

-- another instance for CounterState
instance GenServerState Command Float CounterState where
  handle_call Get = gets (fromIntegral . counter)
  
  handle_cast Inc =
    modify $ \st -> st { counter = counter st + 1 }

-- compiler output
{-
tests/Test/GenServer.hs:22:10:
    Functional dependencies conflict between instance declarations:
      instance GenServerState Command Int CounterState
        -- Defined at tests/Test/GenServer.hs:22:10
      instance GenServerState Command Float CounterState
        -- Defined at tests/Test/GenServer.hs:29:10
-}