creating custom instance of UArray

Question

Suppose I have a simple data type like:

data Cell = Open | Blocked

and I'd like to use a UArray Int Cell. Is there an easy way to do this? Can I somehow reuse the definition for UArray Int Bool?

Answer 1

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.

1. 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
   

2. 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?

3. 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)
   

4. 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.