How could this code enter an infinite loop? aka how to implement a counter in Haskell?

Question

I have this code inside a function with side effects ... -> IO()

let index_player = (-1)
dronePositionByPlayer <- replicateM nb_players $ do

    let index_player = index_player + 1

    dronePositions <- replicateM nb_drones $ do
        input_line <- getLine
        let input = words input_line 
        let dx = read (input!!0) :: Int 
        let dy = read (input!!1) :: Int
        let dronePosition = DronePosition (Position dx dy) index_player
        hPutStrLn stderr $ "position = " ++ show dronePosition
        return (dronePosition)
    return (dronePositions)

When I execute it, while parsing the input data (which is several lines containing each an x and y position) I have this trace on the stardard error output:

position = DronePosition (Position 703 892) Answer: <<loop>> 

Obviously it can read the first position, but then it goes in infinite-loop, probably trying to display the field index_player.

DronePosition and Position are simple algebraic datatypes:

data Position = Position Int Int deriving Show
data DronePosition = DronePosition Position Drone deriving Show

What is ill-formed in my code ?

Answer 1

let index_player = index_player + 1

The above is a recursive definition, resulting in a index_player variable to be computed by adding one recursively -- forever. That does not terminate. It does not refer to the previous variable of the same name declared a few lines above.

If you want a loop, you can adapt e.g.

xs <- forM [0..100] $ \i -> do
   print i
   return (1000 + i)

The above prints all the numbers from 0 to 100, and defines xs to be the list of all the returned numbers, i.e. [1000..1100]. Remember to import Control.Monad to use forM.