How to create unboxed mutable array instance

let's say I've got the following type :

data MyType = Constructor0 | Constructor1 | Constructor2
            deriving (Eq,Show,Enum)

Is there a way to create one of such instances :

MArray (STUArray s) MyType (ST s)
MArray IOUarray MyType IO

For the moment I store everything as Word8 and I make conversion with (wrapped) fromEnum/toEnum, but it doesn't feel right. I need strictness and unboxing because I'm using a large data structure (>1.2Go) in memory, and I can't load it lazily. If I don't find any solution I'm going to re-implement everything in C++, which I prefer to avoid for my current project.

I've asked the question on #haskell but I didn't get a response, maybe it was not the good time of the day to ask.

Solution

The simplest implementation I could think of: just wrap STUArray/IOUArray operations with fromEnum/toEnum.

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

module UnpackedEnumArray (STUEArray, IOUEArray) where

import Control.Monad.ST
import Data.Array.Base
import Data.Array.IO
import Data.Array.ST

data STUEArray s i e = STUEArray { fromSTUEArray :: STUArray s i Int }
instance (Enum e) => MArray (STUEArray s) e (ST s) where
    getBounds = getBounds . fromSTUEArray
    getNumElements = getNumElements . fromSTUEArray
    newArray is = fmap STUEArray . newArray is . fromEnum
    newArray_ = fmap STUEArray . newArray_
    unsafeRead (STUEArray a) = fmap toEnum . unsafeRead a
    unsafeWrite (STUEArray a) i = unsafeWrite a i . fromEnum

data IOUEArray i e = IOUEArray { fromIOUEArray :: IOUArray i Int }
instance (Enum e) => MArray IOUEArray e IO where
    getBounds = getBounds . fromIOUEArray
    getNumElements = getNumElements . fromIOUEArray
    newArray is = fmap IOUEArray . newArray is . fromEnum
    newArray_ = fmap IOUEArray . newArray_
    unsafeRead (IOUEArray a) = fmap toEnum . unsafeRead a
    unsafeWrite (IOUEArray a) i = unsafeWrite a i . fromEnum

Now you can

import UnpackedEnumArray
main = do
    a <- newArray (0,9) Constructor0 :: IO (IOUEArray Int MyType)
    getAssocs a >>= print

Likewise, IArray instances could be trivially written as well.