1 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 hClose h
The same program, with some higher-lever functions:
main = do contents <- readFile "c:\\test.txt" putStrLn contents
2 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"
3 Creating a temporary file
3.1 Writing a filter
Using interact, you can easily do things with stdin and 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]
3.2 Logging to a file