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.