{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Main where

import System.IO.Unsafe (unsafePerformIO)

data Mode = A | B
data Settings (mode :: Mode) where
    SettingsA :: Int -> Settings 'A
    SettingsB :: String -> Settings 'B

deriving instance Show (Settings mode)

-- | Which setting (A or B) I make should depend
-- on whether I choose A or B at compile-time.
makeSettings :: Settings mode -> IO (Settings mode)
makeSettings template = do
  let x = case template of
              SettingsA _ -> SettingsA 123
              SettingsB _ -> SettingsB "456"
  return x

main :: IO ()
main = do
  setA <- makeSettings (SettingsA undefined)
  setB <- makeSettings (SettingsB undefined)
  print setA
  print setB
  putStrLn "done."