Creating custom Unbox types

unbox.hs

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

import qualified Data.Vector.Generic as G
import qualified Data.Vector.Generic.Mutable as M
import qualified Data.Vector.Unboxed as U
import Data.Word (Word64)

newtype Foo = Foo Word64 deriving (Eq, Show)

-- 1. Tell GHC that Foo's Mutable Vector is just a Word64 Mutable Vector
newtype instance U.MVector s Foo = MV_Foo (U.MVector s Word64)

-- 2. Tell GHC that Foo's Immutable Vector is just a Word64 Vector
newtype instance U.Vector Foo = V_Foo (U.Vector Word64)

-- 3. Use GeneralizedNewtypeDeriving to steal the implementations
deriving instance M.MVector U.MVector Foo
deriving instance G.Vector U.Vector Foo

-- 4. Mark it as Unboxable
instance U.Unbox Foo