DB.hs

{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE UndecidableInstances #-}

module DB where

import Data.Aeson
import GHC.Generics

import Types (User)

newtype UserDB = UserDB User deriving newtype Generic deriving stock Show

instance ToJSON UserDB

Types.hs

module Types where

import GHC.Generics

data User = User { name :: String, age :: Int }
  deriving (Show, Generic)

x.cabal

cabal-version: 3.0
name: x
version: 0

library
  build-depends: base, aeson
  exposed-modules:     Types DB
  default-language:    GHC2021