CStruct in haskell (example 1)

CStructExample1.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
module Main where

import GHC.TypeLits
import Data.Proxy

import CStruct

-- this really should be Addr#
data Ptr (k :: Element) = Ptr Int
    deriving (Show,Eq)

plusPtr  :: Ptr k -> Integer -> Ptr v
plusPtr (Ptr p) i = Ptr (p + fromIntegral i)

accessor :: forall fields el child . (KnownNat (Offset 0 fields el), Resolv fields el ~ child)
         => Ptr el -> Ptr child
accessor p = p `plusPtr` (getOffset @el @fields)

type X = 'FStruct
    '[ '("a", 'FInt64)
     , '("b", 'FInt8)
     , '("c", 'FWord32)
     , '("byte", 'FArray 2 'FWord8)
     , '("xyz", 'FArray 12 'FWord64)
     ]

type U = 'FUnion '[ '("w64", 'FWord64)
                  , '("w32", 'FStruct '[ '("high", 'FWord32), '("low", 'FWord32) ])
                  , '("x", X)
                  ]

type Y = 'FStruct
    '[ '("other", 'FInt64)
     , '("myx", X)
     , '("union", U)
     ]

(+>) :: Ptr x -> (Ptr x -> Ptr y) -> Ptr y
(+>) p access = access p

main :: IO ()
main = do
    putStrLn $ "X: " ++ show (getSize @X)
    putStrLn $ "U: " ++ show (getSize @U)
    putStrLn $ "Y: " ++ show (getSize @Y)

    putStrLn $ show (getOffset @X @('[ 'Field "a"]))
    putStrLn $ show (getOffset @Y @('[ 'Field "myx", 'Field "c"]))
    putStrLn $ show (getOffset @Y @('[ 'Field "union", 'Field "x", 'Field "c"]))

    let p  = Ptr 0 :: Ptr Y
        p2 = (accessor @('[ 'Field "myx", 'Field "c"])) p

        p4 = p +> accessor @('Field "myx" ': '[])
        p5 = accessor @('Field "myx" ': '[]) p
        p6 = p +> accessor @('Field "myx" ': 'Field "byte" ': '[ 'Index 0])

    putStrLn $ show p
    putStrLn $ show p2
    putStrLn $ show p6
    return ()