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

[Emil Axelsson]

BTW, this is a case where it may be more convenient to use forM:

  forM ps $ \pix -> do
    particle <- read_grid g pix
    return $ fn particle

(untested...)

forM is just another way of saying (flip mapM).

/ Emil



Andrew Coppin skrev:
> 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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