test.hs

import           Control.Monad.State
import           Control.Monad.ST

-- import           Control.Lens (use, ix, preview, iso, over, both, Lens', Index, IxValue, Ixed)
import           Control.Lens
import           Control.Lens.Operators
import           Control.Lens.TH (makeLenses)

import           Data.Array.ST
import           Data.Array.IArray ((!))
    

data MyState s = MyState { _x :: Word8
                         , _y :: STUArray s Word8 Word8}
makeLenses ''MyState

newtype Computation s a = Computation {unComp :: StateT (MyState s) (ST s) a} 
    deriving (Functor,Applicative,Monad,MonadState (MyState s), MArray (STUArray s) Word8 (StateT (MyState s) (ST s)))


readMemory :: Computation s ()
readMemory = do
    s <- get
    x' <- readArray (s^.y) 0
    x .= x'


    • ‘MArray (STUArray s) Word8 (StateT (MyState s) (ST s))’ is not a unary constraint, as expected by a deriving clause
    • In the newtype declaration for ‘Computation’