Re: [Haskell-cafe] using an external application

On 11/2/07, Petr Hoffmann wrote:

...

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.

I'm puzzled - when I run this on GHCi (v6.4, Windows XP) I get the following outcome *Main>main The process cannot access the file because it is being used by another process. hello *Main> This is certainly the behaviour I would expect - seeing "bye" being printed seems to me to be an error and may even constitute a violation of referential transparency. What implementation of GHC are you using ?

This looks like yet another case of the lazy-I/O goblins.

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