Question

This program produces the output I expect when given an input file of text delimited by \n:

import System.IO

main :: IO ()
main = do h <- openFile "test.txt" ReadMode 
          xs <- getlines h
          sequence_ $ map putStrLn xs

getlines :: Handle -> IO [String]
getlines h = hGetContents h >>= return . lines

By substituting withFile for openFile and rearranging slightly

import System.IO

main :: IO ()
main = do xs <- withFile "test.txt" ReadMode getlines
          sequence_ $ map putStrLn xs

getlines :: Handle -> IO [String]
getlines h = hGetContents h >>= return . lines  

I manage to get no output at all. I’m stumped.

Edit: Not stumped anymore: thanks to one and all for the thoughtful and thought-provoking answers. I did a little more reading in the documentation and learned that withFile can be understood as a partial application of bracket.

This is what I ended up with:

import System.IO

main :: IO ()
main = withFile "test.txt" ReadMode $ \h -> getlines h >>= mapM_ putStrLn 

getlines :: Handle -> IO [String]
getlines h = lines `fmap` hGetContents h

Answer 1

The file is being closed too early. From the documentation:

The handle will be closed on exit from withFile

This means the file will be closed as soon as the withFile function returns.

Because hGetContents and friends are lazy, it won’t try to read the file until it is forced with putStrLn, but by then, withFile would have closed the file already.

To solve the problem, pass the whole thing to withFile:

main = withFile "test.txt" ReadMode $ \handle -> do
           xs <- getlines handle
           sequence_ $ map putStrLn xs

This works because by the time withFile gets around to closing the file, you would have already printed it.