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

[Alfonso Acosta]

I haven't tried to run the code, but my first bet is that, due to the
rank-2 polymorphism of ST, you should use parenthesis instead of $ in
the case of runST.

On Sun, Aug 24, 2008 at 3:25 PM, Andrew Coppin
<andrewcoppin at btinternet.com> wrote:
> 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...
>
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