type_level_nat.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Data.Proxy

import GHC.TypeLits

data A = A
data B = B

type family ArrItem a where
    ArrItem A = '(0xA1, 0xA2)
    ArrItem B = '(0xB1, 0xB2)

printItem :: forall a val1 val2. (ArrItem a ~ '(val1, val2), KnownNat val1, KnownNat val2) => a -> IO ()
printItem _ = do
    let v1 = natVal (Proxy :: Proxy val1)
        v2 = natVal (Proxy :: Proxy val2)
    print (v1, v2)

main :: IO ()
main = do
    printItem A
    printItem B