[Haskell-cafe] using an external application

[Andrew Butterfield]

> On 11/2/07, Petr Hoffmann <spy.hunter at email.cz> 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.
>   
Yes, but not quite what everyone has being addressing....


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