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)