参照透明性を破るSTの使い方

UnsafeST.hs

module UnsafeST where
import Control.Monad.ST (runST, unsafeIOToST)
import Data.Unique (newUnique, hashUnique)

f :: () -> Int
f _ = hashUnique $ runST $ unsafeIOToST newUnique

main :: IO ()
main = do
  print $ f () -- => 1
  print $ f () -- => 2
  print $ f () -- => 3