Counters are initialized every time?

Question

I try to make a simple counter. My counters do not go up however. It seems to me as if they are re-initialized every time by the function \"inc\" or maybe the (n+1) does not

Answer 1

In Haskell data are immutable by default. This means that the c in inc c is always zero.

To get mutable variables in Haskell, you have to ask for them explicitly, i.e. by using IORefs. Using them you could write something like:

import Data.IORef

inc :: IORef Int -> IO ()
inc ref = modifyIORef ref (+1)

main :: IO ()
main = do
  c <- newIORef 0
  f <- newIORef 0
  putStrLn "Starting..."
  conn <- connect "192.168.35.62" 8081
  time $
    forM_ [0..10000] $ \i -> do
      p <- ping conn "ping"
      if p=="pong" 
         then inc c
         else inc f
  c' <- readIORef c
  printf "Roundtrips %d\n" c'