Shows how lazy evaluation of hGetContents interacts with the withFile's automatic handle close.

file_io_example.hs

import Data.Char
import System.IO

readFirstLine = withFile "test.txt" ReadMode (\handle -> do
    line <- hGetLine handle
    return line)

readAndPrintTestFile = withFile "test.txt" ReadMode (\handle -> do
    contents <- hGetContents handle
    -- unless we output contents in some way, it isn't evaluated, and will be empty
    putStrLn contents
    return contents)

readTestFile = withFile "test.txt" ReadMode (\handle -> do
    contents <- hGetContents handle
    let c = map toUpper contents  -- this doesn't evaluate contents!
    return contents)

main = do
    putStrLn "==== Reading first line ===="
    line <- readFirstLine
    putStrLn line
    putStrLn "==== Reading and printing whole file ===="
    contents <- readAndPrintTestFile
    putStrLn contents
    putStrLn "==== Reading whole file ===="
    contents <- readTestFile
    putStrLn contents
    putStrLn "==== Done ===="
    return ()

To run it, do something like:

$ cat - > test.txt 
This is the first line.
This is the second line.
^d
$ ghc --make file_io_example && ./file_io_example