[Haskell-cafe] using an external application

[Petr Hoffmann]

Hi,

I'm solving the following problem - I need to use an external
application - give it the input data and receive its output.
However, when multiple calls are made, the results are not
as expected. The simplified version of the problem is given
below:

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.

Thank you in advance.

Petr