On 25 July 2012 11:40, Bin Shi
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?
If we look at the type of “withFile” we see that it is withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r The important part here is (Handle -> IO r), i.e. a function that takes a handle as an argument and produces a result of IO r. If we de-sugar the do-notation in your first example we get withFile "a.txt" ReadMode (\h -> hGetContents h >>= \c -> putStrLn c) Here we see that the “processing function” passed to withFile is (\h -> hGetContents h >>= \c -> putStrLn c). In your second example, we get the following: withFile "a.txt" ReadMode hGetContents >>= \c -> putStrLn c which is equivalent to (withFile "a.txt" ReadMode hGetContents) >>= \c -> putStrLn c hGetContents is lazy and won't start reading until its needed. This, together with the fact that withFile guarantees that the file handle is closed after it's finished leads to the whole “withFile” call returning "" which is then printed by putStrLn. -- Erlend Hamberg ehamberg@gmail.com