let bGameState :: Behavior t GameState
    bGameState = accumB initialGS $ updateGS <$ eInput

yields the error

 Couldn't match expected type `GameState'
             with actual type `PlayerCommand'
 Expected type: GameState -> GameState
   Actual type: PlayerCommand -> GameState -> GameState
 In the first argument of `(<$)', namely `updateGS'
 In the second argument of `($)', namely `updateGS <$ eInput'

Could not deduce (t ~ t1)
from the context (Frameworks t)
  bound by a type expected by the context:
             Frameworks t => Moment t ()
  at outline.hs:(153,42)-(159,93)
  `t' is a rigid type variable bound by
      a type expected by the context: Frameworks t => Moment t ()
      at outline.hs:153:42
  `t1' is a rigid type variable bound by
       the type signature for bGameState :: Behavior t1 GameState
       at outline.hs:158:8
Expected type: Behavior t1 GameState
  Actual type: Behavior t GameState
In the expression: accumB initialGS $ updateGS <$> eInput
In an equation for `bGameState':
    bGameState = accumB initialGS $ updateGS <$> eInput

makeNetworkDescription :: AddHandler PlayerCommand -> IO EventNetwork
makeNetworkDescription addCommandEvent = compile $ do
   eInput <- fromAddHandler addCommandEvent
   let bCommand = stepper Null eInput
   eCommandChanged <- changes bCommand
   let bGameState :: Behavior t GameState
       bGameState = accumB initialGS $ updateGS <$> eInput
   reactimate $ (n -> appendFile "output.txt" ("Command is " ++ show n)) <$>
   eCommandChanged

(<$)     :: a -> f b -> f a
updateGS :: PlayerCommand -> GameState -> GameState
eInput   :: Event t PlayerCommand

a ~ PlayerCommand -> GameState -> GameState

f b ~ Event t PlayerCommand

f ~ Event t
b ~ PlayerCommand

f a ~ Event t (PlayerCommand -> GameState -> GameState)