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

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...