PFP Laboratory 7

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

Computing PI

• Create a program, that computes aproximation of PI using random numbers - Idea.
• The number of steps will be defined as program's argument.

Monads

• Consider you have a type:

newtype State s a = State { runState :: s -> (s, a) }

Make this type the instance of Monad