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

Thanks for your advice.
I guess promoting While functor to monad and typing countDown Int -> While (Int, Bool) will do. I will try.

Yeah, I made it. But, it is still strange somehow. Anyway, the code can be compiled, runs and works as we expect.

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

instance Monad While where
  return x = While (x, True)
  -- f :: a -> While b
  While (x, True) >>= f = f x
  While (x, False) >>= f = f x

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

main = do
  let result1 = get $ return 1 >>= countDown
  print result1
  let result2 = get $ return 1 >>= countDown >>= countDown >>= countDown
  print result2

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)