creating custom instance of UArray

This answer explains why Vectors are better than Arrays, so I'm going to give you the answer for unboxed vectors.

I did try deriving an MArray and IArray instance for Cell based on the Bool instances, but the Bool instances are quite complicated; it would be at least as ugly as manually deriving an Unbox instance for vectors. Unlike vectors, you also can't just derive Storable and use Storable arrays: you still need the Marray and IArray instances. There doesn't appear to be a nice TH solution yet, so you're better off using vectors for those reasons as well.

There are several ways to do this, some more painful than others.

vector-th-unbox

Pros: Straightforward, much shorter than manually deriving Unbox instances

Cons: Requires -XTemplateHaskell

{-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, TypeFamilies #-}

import Data.Vector.Unboxed
import Data.Vector.Unboxed.Deriving
import qualified Data.Vector.Generic
import qualified Data.Vector.Generic.Mutable

data Cell = Open | Blocked deriving (Show)

derivingUnbox "Cell"
    [t| Cell -> Bool |]
    [| \ x -> case x of
        Open -> True
        Blocked -> False |]
    [| \ x -> case x of
        True -> Open
        False -> Blocked |]

main = print $ show $ singleton Open

Write your own Unbox, M.MVector, and V.Vector instances, plus two data instances

{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}

import qualified Data.Vector.Generic            as V
import qualified Data.Vector.Generic.Mutable    as M
import qualified Data.Vector.Unboxed            as U
import Control.Monad

data Cell = Open | Blocked deriving (Show)

data instance U.MVector s Cell = MV_Cell (U.MVector s Cell)
data instance U.Vector Cell = V_Cell (U.Vector Cell)

instance U.Unbox Cell

{- purloined and tweaked from code in `vector` 
   package that defines types as unboxed -}
instance M.MVector U.MVector Cell where
  {-# INLINE basicLength #-}
  {-# INLINE basicUnsafeSlice #-}
  {-# INLINE basicOverlaps #-}
  {-# INLINE basicUnsafeNew #-}
  {-# INLINE basicUnsafeReplicate #-}
  {-# INLINE basicUnsafeRead #-}
  {-# INLINE basicUnsafeWrite #-}
  {-# INLINE basicClear #-}
  {-# INLINE basicSet #-}
  {-# INLINE basicUnsafeCopy #-}
  {-# INLINE basicUnsafeGrow #-}

  basicLength (MV_Cell v) = M.basicLength v
  basicUnsafeSlice i n (MV_Cell v) = MV_Cell $ M.basicUnsafeSlice i n v
  basicOverlaps (MV_Cell v1) (MV_Cell v2) = M.basicOverlaps v1 v2
  basicUnsafeNew n = MV_Cell `liftM` M.basicUnsafeNew n
  basicUnsafeReplicate n x = MV_Cell `liftM` M.basicUnsafeReplicate n x
  basicUnsafeRead (MV_Cell v) i = M.basicUnsafeRead v i
  basicUnsafeWrite (MV_Cell v) i x = M.basicUnsafeWrite v i x
  basicClear (MV_Cell v) = M.basicClear v
  basicSet (MV_Cell v) x = M.basicSet v x
  basicUnsafeCopy (MV_Cell v1) (MV_Cell v2) = M.basicUnsafeCopy v1 v2
  basicUnsafeMove (MV_Cell v1) (MV_Cell v2) = M.basicUnsafeMove v1 v2
  basicUnsafeGrow (MV_Cell v) n = MV_Cell `liftM` M.basicUnsafeGrow v n

instance V.Vector U.Vector Cell where
  {-# INLINE basicUnsafeFreeze #-}
  {-# INLINE basicUnsafeThaw #-}
  {-# INLINE basicLength #-}
  {-# INLINE basicUnsafeSlice #-}
  {-# INLINE basicUnsafeIndexM #-}
  {-# INLINE elemseq #-}

  basicUnsafeFreeze (MV_Cell v) = V_Cell `liftM` V.basicUnsafeFreeze v
  basicUnsafeThaw (V_Cell v) = MV_Cell `liftM` V.basicUnsafeThaw v
  basicLength (V_Cell v) = V.basicLength v
  basicUnsafeSlice i n (V_Cell v) = V_Cell $ V.basicUnsafeSlice i n v
  basicUnsafeIndexM (V_Cell v) i = V.basicUnsafeIndexM v i
  basicUnsafeCopy (MV_Cell mv) (V_Cell v) = V.basicUnsafeCopy mv v
  elemseq _ = seq

main = print $ show $ U.singleton Open

Wasn't that fun?

Create a Storable instance and use Data.Vector.Storable instead.

Pros: No TH, and relatively simple instance

Cons: The instance is less obvious than the TH definition. Also, whenever you ask a SO question about Storable vectors, someone will inevitably ask why you aren't using Unboxed vectors, though no one seems to know why Unboxed vectors are better.

For a data:

{-# LANGUAGE ScopedTypeVariables #-}

import Control.Monad
import Data.Vector.Storable
import Foreign.Storable

import GHC.Ptr
import GHC.Int

-- defined in HsBaseConfig.h as 
-- #define HTYPE_INT Int32
type HTYPE_INT = Int32

data Cell = Open | Blocked deriving (Show)

instance Storable Cell where
 sizeOf _          = sizeOf (undefined::HTYPE_INT)
 alignment _       = alignment (undefined::HTYPE_INT)
 peekElemOff p i   = liftM (\x -> case x of 
                        (0::HTYPE_INT) -> Blocked
                        otherwise -> Open) $ peekElemOff (castPtr p) i
 pokeElemOff p i x = pokeElemOff (castPtr p) i $ case x of
    Blocked -> 0
    Open -> (1 :: HTYPE_INT)

main = print $ show $ singleton Open

Or for a newtype:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Data.Vector.Storable as S
import Foreign.Storable

newtype Cell = IsOpen Bool deriving (Show)

main = print $ show $ S.singleton (Foo True)

Unbox instances for newtype

This doesn't directly apply to your question since you don't have a newtype, but I'll include it for completeness.

Pros: No TH, no code to write, still using Unboxed vectors for the haters

Cons: None?

{-# LANGUAGE GeneralizedNewtypeDeriving, 
             StandaloneDeriving, 
             MultiParamTypeClasses #-}

import Data.Vector.Generic as V
import Data.Vector.Generic.Mutable as M
import Data.Vector.Unboxed as U

newtype Cell = IsOpen Bool deriving (Unbox, Show)
deriving instance V.Vector U.Vector Cell
deriving instance M.MVector U.MVector Cell

main = print $ show $ U.singleton (IsOpen True)

EDIT

Note that this solution currently isn't possible in GHC 7.8.

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