Reading from a file
The System.IO library contains the functions needed for file IO. The program below displays the contents of the file c:\test.txt.
import System.IO main = do h <- openFile "c:\\test.txt" ReadMode contents <- hGetContents h putStrLn contents
Note: It is important never to call
hClose on a file-handle which has had
hGetContents run on it already. The file handle is in a semi-closed state, and will be closed when the resulting string is garbage collected. Closing it manually may result in a random truncation of the input.
The same program, with some higher-level functions:
main = do contents <- readFile "c:\\test.txt" putStrLn contents
Writing to a file
The following program writes the first 100 squares to a file:
-- generate a list of squares with length 'num' in string-format. numbers num = unlines $ take num $ map (show . \x -> x*x) [1..] main = do writeFile "test.txt" (numbers 100) putStrLn "successfully written"
This will override the old contents of the file, or create a new file if the file doesn't exist yet. If you want to append to a file, you can use
Creating a temporary file
TODO: abstract via 'withTempFile', handle exception
import System.IO import System.Directory main = do tmpDir <- getTemporaryDirectory (tmpFile, h) <- openTempFile tmpDir "foo" hPutStr h "Hello world" hClose h removeFile tmpFile
Writing a filter
Using interact, you can easily do things with stdin and stdout.
interact takes a function from
string and applies it lazily to the input from stdin, writing the output to stdout.
A program to sum up numbers:
main = interact $ show . sum . map read . lines
A program that adds line numbers to each line:
main = interact numberLines numberLines = unlines . zipWith combine [1..] . lines where combine lineNumber text = concat [show lineNumber, " ", text]
Logging to a file