Hi,
From the withFile doc (http://hackage.haskell.org/packages/archive/haskell2010/latest/doc/html/Sys tem-IO.html#v:withFile):
The handle will be closed on exit from withFile, whether by normal termination or by raising an exception. Your program is effectively this one main' :: IO () main' = do h <- openFile "a.txt" ReadMode c <- hGetContents h hClose h putStrLn c Now we need to check what value c has in this situation. It is actually dependent upon how much c has been read when the file is closed. (http://hackage.haskell.org/packages/archive/haskell98/latest/doc/html/IO.ht ml#v:hGetContents) Once a semi-closed handle becomes closed, the contents of the associated list becomes fixed. (You will need to read all of the documentation to get the context -- what 'semi-closed' means, etc.) In the above program, none of the input has been read when h is closed so 'c' is empty. Disconcerted? hGetContents (and getContents) are probably like no other function you will encounter in the standard Haskell libraries. I think they are there because it is extremely convenient for certain kinds of widely used idioms but that convenience comes at a price. But these are Haskell outliers. With less pitfalls I would recommend using getContents where possible. Chris -----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Bin Shi Sent: 25 July 2012 11:00 To: haskell-cafe@haskell.org Subject: [Haskell-cafe] A Question about withFile Hello, I wrote a simple test program as main = do withFile "a.txt" ReadMode (\h -> do c <- hGetContents h putStrLn c) then I got my expected results: I'm a.txt but if I changed to main = do c <- withFile "a.txt" ReadMode hGetContents putStrLn c I got just a empty line. Where am I wrong? Thanks. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe