Typeable-based natVal.

{-# LANGUAGE KindSignatures, DataKinds #-}

import GHC.TypeLits (Nat)
import Data.Typeable

-- I didn't require KnownNat!
natVal :: Typeable n => proxy (n :: Nat) -> Int
natVal n = read . show $ typeRep n


main = do
    putStrLn $ show $ natVal (undefined :: Proxy 257)