{-# LANGUAGE TemplateHaskell #-}
module Main where

import Data.IORef (IORef, atomicModifyIORef', newIORef)

import InitOnce


globalRef :: IORef Int
globalRef = $$(once [|| newIORef 0 ||])

-- wrong :: Typeable a => IORef [a]    -- does not typecheck: no instance Typeable a
-- wrong = $$(once [|| newIORef [] ||])

{-# NOINLINE foo #-}
foo :: IO String
foo = atomicModifyIORef' globalRef (\i -> (i + 1, show i))

main :: IO ()
main = do
  putStrLn =<< foo  -- 0
  putStrLn =<< foo  -- 1