unsafeInterleaveIOがまずいのがわかるコード辺

gistfile1.txt

$ runhaskell UnsafeIO.hs
unsafeHello #1
unsafeHello #2
Hello, World
Hello, World #2
()

UnsafeIO.hs

module Main where
import System.IO.Unsafe

hello = do
  unit <- putStrLn "Hello, World"
  putStrLn "Hello, World #2" 
  return unit

unsafeHello = unsafeInterleaveIO hello

main = do
  putStrLn "unsafeHello #1"
  unsafeHello                -- なにも表示されない
  putStrLn "unsafeHello #2"
  unsafeHello >>= print      -- 表示される