# [Newbie] Streams and IO

Christian Hofer christian.hofer@gmx.de
Sun, 10 Aug 2003 19:08:59 +0200

```Hello,

I am completely new to functional programming, am just reading through
SOE (great book so far!) and try to understand streams in
client-server-interactions. Therefore I wrote the following program
(which has nothing to do with the book exercises), which is intended to
read integers from stdin and to write the sum of the inputs so far to
stdout.

When I start it (in Hugs), I can enter numbers, but don't get back the
sum, but every second time a zero, every forth time the value I just
entered and every forth time the sum of the two values I entered the latest.

Could s.o. explain me, what is going wrong? I can't figure out, what is
happening here. (I know that there are easier solutions to the problem
than using streams, but the latter was my intention for learning.)

Thanks,
Christian

module Main where

main = sequence reqs

type Request = IO Int
type Response = IO Int

reqs :: [Request]
reqs = client resps
resps :: [Response]
resps = server reqs

client :: [Response] -> [Request]
client (y:ys) = (do toPrint <- y
putStr ("So far: " ++ show toPrint ++ "\nEnter
number: ")
line <- getLine
return (parseToInt line)) : client ys

server :: [Request] -> [Response]
server = scanl helper (return 0)
where helper :: IO Int -> IO Int -> IO Int
helper sofar x = do a <- x
sf <- sofar
return (a + sf)

parseToInt :: String -> Int
parseToInt str = foldl nextDigit 0 (intList str)
where intList :: String -> [Int]
-- only for positive numbers so far
intList = map (\x -> fromEnum x - 48)
nextDigit :: Int -> Int -> Int
nextDigit a b = a * 10 + b

```