Ok, the problem was the Monad, which I still don't get completely, because I continue like this: data Field = W|B|H|D deriving (Eq,Show) pas :: ST s (STArray s (Int, Int) Field) -> ST s (STArray s (Int, Int) Field) pas b = do writeArray b (1,1) W And still getting: Couldn't match expected type `STArray s (Int, Int) Field' against inferred type `Field' Thanks. On Feb 21, 2010, at 5:33, Ben Millwood wrote:

2010/2/21 Vojtěch Knyttl :

...

Hello,

I am trying to create STArray with newListArray like this: la x y = newListArray ((1,1),(x,y)) [(x'+y') | x' <- [1..x], y' <- [1..y]]

– but it does not work: No instance for (MArray a Field m)

Notice that newListArray has a monadic return type:

newListArray :: (MArray a e m, Ix i) => (i, i) -> [e] -> m (a i e)

with MArray a e m requiring Monad m. So newListArray returns your STArray in a monad. That pretty much has to be the ST monad, but that's not in scope, so you need to import Control.Monad.ST before the MArray (STArray s) e (ST s) instance is usable.

...

I tried to define the type like this, but it would not work either: la :: Int -> Int -> STArray (Int,Int) Field

The kind for STArray here is slightly off. STArray needs three type parameters, not two. This is linked to the fact that ST is not a monad, but (ST s) is - the extra parameter is essential to ST, to ensure the mutability doesn't "leak out". You probably want:

la :: Integer -> Integer -> ST s (STArray s (Integer, Integer) Field)