label-modifier.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}

module Data.Aeson.GenericFields (type (<?>)) where

import Data.Proxy
import GHC.Generics
import GHC.TypeLits

type a <?> name = a

class GLabelModifier a where
  glabelModifier :: [(String, String)]

instance GLabelModifier constructors => GLabelModifier (D1 meta constructors) where
  glabelModifier = glabelModifier @constructors

instance (GLabelModifier l, GLabelModifier r) => GLabelModifier (l :+: r) where
  glabelModifier = glabelModifier @l <> glabelModifier @r

instance GLabelModifier selectors => GLabelModifier (C1 meta selectors) where
  glabelModifier = glabelModifier @selectors

instance (GLabelModifier l, GLabelModifier r) => GLabelModifier (l :*: r) where
  glabelModifier = glabelModifier @l <> glabelModifier @r

instance GLabelModifier (S1 meta sel) where
  glabelModifier = []

instance {-# OVERLAPS #-} (KnownSymbol old, KnownSymbol new) => GLabelModifier (S1 (MetaSel (Just old) _m1 _m2 _m3) (Rec0 (a <?> new))) where
  glabelModifier = [(symbolVal @old Proxy, symbolVal @new Proxy)]

labelModifier :: forall a. GLabelModifier (Rep a) => [(String, String)]
labelModifier = glabelModifier @(Rep a)

data X = X {a :: Int <?> "aasd", b :: String} deriving (Generic)

x = labelModifier @X