FP Laboratory 12/en

Working with files - monad IO

• Create a program that reads a file and number it's lines and writes it on the screen.

For the solution look at: openFile :: FilePath -> IOMode -> IO Handle

The Handle can used to work with a file. The function to read a content: hGetContents :: Handle -> IO String

At the end, you should close the opened file handle: hClose :: Handle -> IO ()

Alternatively you can use : readFile :: FilePath -> IO String

import System.IO
import Control.Exception

main = do fromHandle <- opf "Read from: " ReadMode
          contents <- hGetContents fromHandle
          putStr (numberLines contents)
          hClose fromHandle

opf :: String -> IOMode -> IO Handle
opf prompt mode = do putStr prompt
                     name <- getLine  
                     catch (openFile name mode )
                           (\e -> do putStr ("Can't open "++name++":"++show (e :: IOException) ++"\n")
                                     opf prompt mode)

numberLines::String -> String
numberLines text = let l = lines text
                       numbered = zip [1..] l
                       result = [show x ++".\t"++content++"\n" | (x, content)<-numbered]
                   in concat result