You need -fglasgow-exts to get the type signatures. you can get around it with: instance MArray IOUArray Foo IO where unsafeRead arr = do arr2 <- myCast ... where myCast :: IOUArray i Foo -> IO (IOUArray i Int) myCast = unsafeCastIOUArray
There is only a small problem: there is no such function as unsafeRead or unsaferead, at at least not in GHC (I saw unsafeThaw, but that also didn't work).
oh come on, do you really believe that? Prelude Data.Array.MArray> :info MArray -- MArray is a class class (HasBounds a, Monad m) => MArray a :: (* -> * -> *) e m :: (* -> *) where { newArray :: forall i. (Ix i) => (i, i) -> e -> m (a i e) {- has default method -}; newArray_ :: forall i. (Ix i) => (i, i) -> m (a i e) {- has default method -}; Data.Array.Base.unsafeRead :: forall i. (Ix i) => a i e -> Int -> m e; Data.Array.Base.unsafeWrite :: forall i. (Ix i) => a i e -> Int -> e -> m (); } perhaps importing Data.Arary.Base might help :) - hal