Storing an Enum type in an unboxed Vector - haskell

Question

Suppose I have something like this:
data Colour = Red | Blue | Green
deriving (Eq, Ord, Enum, Bounded, Read, Show)
And I want to have an unboxed Vector of Colours. I obviously cannot do this directly (because Colour isn't an instance of Unbox), but I also can't tell how I would write the Unbox instance for Colour. The the documentation for Unbox doesn't seem to say how you make something an instance of it (or at least, not in a way I understand).

Answer 1

One approach is to use Data.Vector.Unboxed.Deriving, which uses template Haskell to define the correct instances for the new types in terms of existing types with Unbox instances.
{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, TemplateHaskell #-}
module Enum where
import qualified Data.Vector.Unboxed as U
import Data.Vector.Generic.Base
import Data.Vector.Generic.Mutable
import Data.Vector.Unboxed.Deriving
import Data.Word
data Colour = Red | Blue | Green
deriving (Eq, Ord, Enum, Bounded, Read, Show)
colourToWord8 :: Colour -> Word8
colourToWord8 c =
case c of
Red -> 0
Blue -> 1
Green -> 2
word8ToColour :: Word8 -> Colour
word8ToColour w =
case w of
0 -> Red
1 -> Blue
_ -> Green
derivingUnbox "Colour"
[t| Colour -> Word8 |]
[| colourToWord8 |]
[| word8ToColour |]
test n = U.generate n (word8ToColour . fromIntegral . (`mod` 3))
Of course this wastes space in this case because we only use 2 of the 8 bits in Word8.