LYAH: Ch 9, I/O

file.txt

Hey there
line two
whoa

main.hs

import System.Random
import System.IO
import System.Environment
import Control.Monad (forever)

-- forever :: Monad m => m a -> m b
printForever :: (Show a) => a -> IO b
printForever a = forever (print a)

printRandomNumber :: IO ()
printRandomNumber = do
    n <- randomIO :: IO Int
    print n

-- Unix `wc` program: `interact` reads all of STDIN and runs the provided
-- function (String -> String) over it
wc :: IO ()
wc = interact (\s -> (show $ length s) ++ "\n")

fileStuff :: IO String
fileStuff = do
    putStrLn ""
    contents <- readFile "file.txt"
    putStrLn ">> Contents of file.txt:"
    putStrLn contents
    writeFile "hey.txt" "Hi everyone!"
    putStrLn ">> Reading out hey.txt, which we just wrote:"
    handle <- openFile "hey.txt" ReadMode -- gives us a Handle
    putStrLn ">> First line of hey.txt:"
    hGetLine handle
    putStrLn ">> Rest of hey.txt:"
    hGetContents handle

-- How to desugar:
-- http://www.haskellforall.com/2014/10/how-to-desugar-haskell-code.html
printRandomNumber' :: IO ()
printRandomNumber' = (randomIO :: IO Int) >>= (\n -> print n)

printRandomNumber'' :: IO ()
printRandomNumber'' = (randomIO :: IO Int) >>= print

-- >>= :: (Monad m) => m a -> (a -> m b) -> m b
-- Above: IO is the monad `m`
-- IO a -> (a -> IO b) -> IO b
-- randomIO is of type IO a, here cast to IO Int, so a is Int:
-- IO Int -> (Int -> IO b) -> IO b
-- print is (Int -> IO b), since it takes anything Showable and gives `IO ()`:
-- IO Int -> (Int -> IO ()) -> IO ()
-- Thus the return type is IO ()

notes.txt

* Haskell I/O is lazy like everything else
* New I/O control functions: `forever`, `interact`
* New I/O actions: `openFile`, `readFile`, `writeFile`, `withFile`, `appendFile`
* File handle functions: `hGetContents`, `hGetLine`, etc
* Random numbers: `random`, `randomR`, `mkStdGen`, `getStdGen`

setup

#!/bin/sh

cabal install random