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.

module Main (main) where
import System.Random
import System.Environment

myCycle :: Integer -> Integer -> IO Integer
myCycle i c
  | i == 0 = return c
  | otherwise = do 
    x <- randomRIO (0, 1.0) :: IO Double
    y <- randomRIO (0, 1.0) :: IO Double
    myCycle (i-1) (if sqrt (x*x + y*y) <= 1 then c+1 else c)

main :: IO ()
main = do 
    [arg] <- getArgs
    let number = read arg::Integer
    inside <- myCycle number 0
    let my_pi = (fromIntegral (4*inside) / fromIntegral number) :: Double
    print my_pi

Monads

• Make following types instances of the type class Monad.

data MyMaybe a = MyJust a | MyNothing
data MyList a = MyList [a]
data MyEither a b = MyLeft a | MyRight b