monad writer的函数不返回结果

Question

我正在尝试实施fixed point功能，如下所示

fixedPoint  :: (Fractional a,Ord a ) => (a  -> a ) -> a -> a 
fixedPoint f initialGuess= let x= f initialGuess
                       in if (closeEnough initialGuess x) 
                       then x
                       else fixedPoint f x 

这个函数在调用fixedPoint cos 1并返回0.7390851699445544时效果很好但是，我想记录函数完成的步骤并使用writer monad将其添加到字符串中，调整后的函数是按照以下

fixedPoint'  :: (Fractional a,Ord a,Show a) => (a -> a ) -> a -> Writer [String] a 
fixedPoint' f initialGuess= 
let x = f initialGuess 
in 
if (closeEnough initialGuess x ) then
  do
    tell ["Ending with" ++ show initialGuess]
    return x
else
  do
    result <- fixedPoint' f x
    tell ["Trying Guess" ++ show initialGuess]
    return result

调用函数mapM_ putStrLn $ snd $ runWriter (fixedPoint' cos 1)时返回空元组