[Haskell-cafe] Why doesn't this work?

[Andrew Coppin]

colour_grid :: (Particle -> IO ()) -> Grid ph -> IO ()
colour_grid fn g = sequence_ $ runST $ do
  ps <- grid_coords g

  mapM
    (\pix -> do
      particle <- read_grid g pix
      return $ fn particle
    )
    ps

When I attempt to run this, GHCi just gives me a very cryptic type 
checker error. I can't figure out what's wrong here. As far as I can 
tell, this should run...