On Fri, 2 Nov 2007, Petr Hoffmann wrote:

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

You fell into the trap, that 'readFile' works lazily, that is, data is only read when needed. If o1 is not completely evalutated until the second write to output.txt, it will not be read correctly from the disk. I think it was not a good choice to make the lazy version of 'readFile' the default. I think it would have been better to provide a strict 'readFile' and another 'readFileLazy' with warnings in the documentation.

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.

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

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.

Other people have explained to the OP why unsafe lazy IO is breaking his code. Yet another piece of evidence, in my opinion, that unsafe-lazy-by-default is the wrong basic API. Provide that API as an option, sure. But as a default? Not IMO. Jules