While Functor prototype failure

While.hs

newtype While a = While { get :: (a, Bool) }

instance Functor While where
  fmap f (While (a, True)) = let (a', b) = f a in While (a', b)
  fmap _ (While (a, False)) = While (a, False)

countDown :: Int -> (Int, Bool)
countDown i =
  let i' = i - 1
  in 
    if i' < 0 then (i, False) else (i', True)

main = do
  let result = get $ While (1, True) `fmap` countDown `fmap` countDown 
  print result

The boolean value isn't really used. This is practically just Identity monad.

BTW, you can change the implementation of coundDown to:

countDown :: Int -> While Int
countDown i = do
  let i' = i - 1
  While (i, i' >= 0)

which can be even simplified to

countDown :: Int -> While Int
countDown i = While (i, i >= 1)

Could you explain what you are trying to do?

Thanks.
One, You are right! This is definitely Identity monad.
Two, You may be wrong. Your countDown implementations are not the same as mine because my version returns While monad with i' if the condition is True whereas with i if False. The difference is your version always returns with i.

You're right. I didn't notice the ' after one of the i:s.

You could however write

While $ if i' < 0 
  then (i, False)
  else (i', True)