{-# LANGUAGE BlockArguments, RequiredTypeArguments #-}

module LocalSigsST where

import Data.STRef
import Control.Monad.ST

-- Haskell2010
doit1 :: Int
doit1 = runST do
  x <- newSTRef 0

  let mogrify :: STRef s Int -> ST s ()
      mogrify y = modifySTRef y succ

  mogrify x
  readSTRef x

-- ScopedTypeVariables
doit2 :: Int
doit2 = runST do
  (x :: STRef s Int) <- newSTRef 0

  let mogrify :: ST s ()
      mogrify = modifySTRef x succ

  mogrify
  readSTRef x

-- RequiredTypeArguments
doit3 :: Int
doit3 = runST' \s -> do
  x <- newSTRef 0

  let mogrify :: ST s ()
      mogrify = modifySTRef x succ

  mogrify
  readSTRef x

runST' :: (forall s -> ST s a) -> a
runST' f = runST (unreq f)
 where
  unreq :: forall t a. (forall s -> ST s a) -> ST t a
  unreq f = f t