PFP Laboratory 7
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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