I thank people for the notes. People write that the example without unsafePerformIO is desirable. So I present the improved question and improved, "pure" code. ---------------------------------------------------------------------- I need to organize a string interface for a Haskell function Main.axiom and a C program fifoFromA.c via a pair of named pipes (in Linux, UNIX). The pipes are created before running, by the commands > mkfifo toA > mkfifo fromA Main.axiom outputs a string to toA and inputs the respond string from fromA as the result. fifoFromA inputs a string from toA, converts it to the string resStr, outputs resStr to fromA. Main.axiom must be able to be applied in a loop, and there must be avoided repeated opening of a file/channel in a loop. As an example, the string conversion in fifoFromA.c is put the conversion of each character to the lower case: -------------------------- fifoFromA.c ------------------------------ #include <string.h> #include <stdio.h> #include <fcntl.h> #define BOUND 64 int main() { int toA, fromA, i, numread; char buf[BOUND]; toA = open("toA", O_RDONLY); fromA = open("fromA", O_WRONLY); for (;;) { numread = read(toA, buf, BOUND); buf[numread] = '\0'; // printf("A: Read from toA: %s\n", buf); i = 0; // convert the string to the lower case while (i < numread) {buf[i] = tolower(buf[i]); i++;} write(fromA, buf, strlen(buf)); } } ----------------------------------------------------------------------- For the "to-A" part writen in C (instead of Haskell), this interface loop works all right. With Haskell, I manage to process only the first string in the loop, and then it ends with an error. Main.hs is given below. Can you, please, fix the code? Please, copy the response to mechvel@botik.ru Thanks, ------ Sergei mechvel@botik.ru ------------------------------------------------------------------------ import System.IO (IOMode(..), IO(..), Handle, openFile, hPutStr, hGetLine, hFlush) dir = showString "/home/mechvel/ghc/axiomInterface/byLowerLevel/" toA_IO = openFile (dir "toA") WriteMode :: IO Handle -- used as global values fromA_IO = openFile (dir "fromA") ReadMode -- axiomIO :: String -> IO String axiomIO str = do toA <- toA_IO fromA <- fromA_IO hPutStr toA str hFlush toA hGetLine fromA ---------------------------------------------- Example of usage -- main = do putStr "str1 --> " str1' <- axiomIO str1 putStr (str1' ++ "\n") putStr "str2 --> " str2' <- axiomIO str2 putStr (str2' ++ "\n") where n = 9 str0 = "ABC" strings = [str0 ++ (show i) | i <- [1 .. n]] str1: str2: _ = strings ------------------------------------------------------------------ I use Glasgow Haskell ghc-7.01. Build: > gcc -o fifoFromA fifoFromA.c > ghc --make Main Running: first, command > ./fifoFromA on terminal-2, then command > ./Main on terminal-1. And it processes only the first string: terminal-1: m> ./Main str1 --> abc1 str2 --> Main: /home/mechvel/m/toA: openFile: does not exist (No such device or address)