ADTをUnboxed Vectorにして、Lens経由でアクセスする

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

import           Control.Lens
import           Data.Vector.Generic.Lens
import qualified Data.Vector.Unboxed          as VU
import           Data.Vector.Unboxed.Deriving

data FluidCell a
  = FluidCell
    { _mass   :: !a
    , _energy :: !a
    , _volume :: !a
    }
    deriving (Eq, Show)

makeLenses ''FluidCell

derivingUnbox "FluidCell"
  [t| VU.Unbox a => FluidCell a -> (a, a, a) |]
  [e| \(FluidCell x y z) -> (x, y, z) |]
  [e| \(x, y, z) -> FluidCell x y z |]

main :: IO ()
main = do
  let v = VU.fromList [FluidCell (1 :: Int) 2 3, FluidCell 4 5 6] :: VU.Vector (FluidCell Int)
  print v
  -- > fromList [FluidCell {_mass = 1, _energy = 2, _volume = 3},FluidCell {_mass = 4, _energy = 5, _volume = 6}]
  print $ v ^._head . mass
  -- > 1
  print $ _head . mass .~ 9 $ v
  -- > fromList [FluidCell {_mass = 9, _energy = 2, _volume = 3},FluidCell {_mass = 4, _energy = 5, _volume = 6}]