ezyang@ezyang:~$ cat Test.hs
    import System.IO.Unsafe
    unUnsafePerformIO = return
    main = do
      let a = unUnsafePerformIO (unsafePerformIO (putStrLn "boom"))
      a
      a
    ezyang@ezyang:~$ runghc Test.hs
    ezyang@ezyang:~$