FirstField.hs

{-# LANGUAGE DataKinds, TypeOperators, GADTs, TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
module FirstField where

import Generics.SOP

class HasHead (y :: *) (xs :: [*]) where
  hhead :: NP I xs -> K y xs
instance (x ~ y) => HasHead y (x ': xs) where
  hhead (I x :* _) = K x

firstField :: forall a x . (Generic a, All (HasHead x) (Code a)) => a -> x
firstField x = unI (hcollapse (hcliftA p hhead (unSOP (from x))))
  where
    p = Proxy :: Proxy (HasHead x)