On Tue, Dec 9, 2014 at 2:21 PM, Zoran Plesivčak
I've encountered unexplainable of "withFile" function. Consider "example.hs" program:
import System.IO
main = do cnts <- withFile "example.hs" ReadMode $ (\h -> do res <- hGetContents h --putStr res return res) putStr cnts
When commented-out line is like that, program doesn't write out anything to the STDOUT. If "--" (commend characters) are removed, program writes out contents of "example.hs" two times.
I'm not an expert, but I believe you're problem is lazy IO. hGetContents doesn't actually read the file until something consumes it. In the version with the first putStr commented out, that doesn't happen until you do the putStr outside the withFile - by which time the file handle has been closed, so nothing ever gets read. With the putStr inside the withFile, that consumes the contents before the file is closed, so it gets read and then returned, where the second putStr outputs it a second time. If I'm right, you need to force the evaluation of res before the return, with something like "res seq return res" instead of just "return res". Or use the BangPatterns extension and then write "!res <- hGetContents h"