mfix.hs

{-# LANGUAGE RecursiveDo #-}

f = do rec a <- do a <- b
                   putStrLn "Hello"
                   return a
           b <- do return $ return a :: IO (IO ())
       return a

f' = do rec b <- do return $ return a :: IO (IO ())
            a <- do a <- b
                    putStrLn "Hello"
                    return a
        return a


g = do rec a <- do a <- return a
                   putStrLn "Hello"
                   return a
           b <- return $ return a :: IO (IO ())
       return a

{-
*Main> g
Hello
*Main> f
*** Exception: cyclic evaluation in fixIO
*Main> f'
Hello
-}