records.hs

{-# language DataKinds #-}
{-# language DuplicateRecordFields #-}
{-# language OverloadedRecordDot #-}

import GHC.Records

data Person = Person
  { name :: String
  , pet :: Pet
  , partner :: Maybe Person
  }

class Display a where
  display :: a -> String

instance Display Person
  where display p = "Person { name = \"" <> p.name <> "\"}"

data Pet = Pet
  { name :: String
  }
  deriving Show

instance Display Pet where
  display = show

dino = Pet {name = "Dino"}
wilma = Person {name = "Wilma", pet = dino, partner = Just fred}
fred = Person {name = "Fred Flintstone", pet = dino, partner = Just wilma}

data T where
  A :: Display a => a -> T

s :: T -> String
s (A a) = display a

main = do
  print $ map (s . snd) [("dino", A dino), ("wilma", A wilma), ("fred", A fred)]
  print dino
  print $ display wilma
  print $ display fred
  putStrLn fred.name
  putStrLn $ (.name) fred
  putStrLn $ fred.pet.name
  putStrLn $ (.name) $ (.pet) fred
  putStrLn $ getField @"name" $ getField @"pet" $ fred