On 11/2/07, Petr Hoffmann
import System.Cmd main = do System.Cmd.system "echo hello >output.txt" -- use the external application to create an output file o1 <- readFile "output.txt" System.Cmd.system "echo bye >output.txt" -- the second call to the external application o2 <- readFile "output.txt" putStr o1 -- "hello" expected, but "bye" printed return 0
Can you please give me some hint to solve this problem? I'm a beginning haskell developer and I'm still a bit confused by the IO monad.
This looks like yet another case of the lazy-I/O goblins. The "readFile" function uses evil magic to avoid actually performing any I/O until the contents are actually used. In your case, I suspect that by the time "o1" is used -- i.e. in the "putStr" call -- the file contents have already changed, so the lazy I/O reads the new contents without complaining. The solution would be to use a version of "readFile" that works in a stricter way, by reading the file when it's told to, but I don't have an implementation handy. Stuart