Done!!!!

dhall.hs

import           Dhall           (Interpret (..), Inject(..))
import qualified Dhall           as D
import           Dhall.Core (pretty)
import qualified Data.Text as T
import GHC.Generics
import Control.Monad.Trans.State.Strict (evalState)
import Data.Functor.Contravariant

data Person = Person {
      pName :: !Text
    , pAge  :: !Natural
    } deriving (Eq, Show, Generic)

instance Interpret Person where
    autoWith _ = D.record $ 
        Person <$> D.field "name" D.auto 
               <*> D.field "age" D.auto

-- Define instance using generics
instance Inject Person where
    injectWith _ = contramap GHC.Generics.from (evalState (D.genericInjectWith option) 1)
      where
        option = D.defaultInterpretOptions {D.fieldModifier = lowerHead . T.drop 1}

-- | I knowwww it's partial function, but I don't think there would be an field with empty key in Dhall files
-- so I'd presume this is safe
lowerHead :: T.Text -> T.Text
lowerHead str = T.toLower $ T.take 1 str <> T.drop 1 str

-- > let p = Person "Hiroto" 10000
-- > pretty $ embed myInject p
-- > "{ name = "hiroto", age = 29 }"
testRoundTrip :: (Inject a, Interpret a, Eq a) => a -> IO Bool
testRoundTrip p = do
    person <- D.input D.auto $ pretty $ D.embed D.inject p
    return $ p == person