Taking Notes from http://learnyouahaskell.com

IO

你能把 I/O action 想成是一个长了脚的盒子，它会跑到真实世界中替你做某些事
打开盒子的唯一办法就是用 <-
一个 I/O action 会在我们把它绑定到 main 这个名字并且运行程序的时候触发
do 表示法将所有 I/O action 绑成一个

Hello World

hello world 来的晚了点：）

1	main = putStrLn "hello, world"

ghci> :t putStrLn
putStrLn :: String -> IO ()
ghci> :t putStrLn "hello, world"
putStrLn "hello, world" :: IO ()

return an I/O action
it container () () means return nothing

1 2	ghci> :t getLine getLine :: IO String

Let Binding

import Data.Char
main = do
    putStrLn "What's your first name?"
    firstName <- getLine
    putStrLn "What's your last name?"
    lastName <- getLine
    let bigFirstName = map toUpper firstName
        bigLastName = map toUpper lastName
    putStrLn $ "hey " ++ bigFirstName ++ " " ++ bigLastName ++ ", how are you?"

let bindings in expression (in expression is not required)

Example

main = do
    line <- getLine
    if null line
        then return ()
        else do
            putStrLn $ reverseWords line
            main
reverseWords :: String -> String
reverseWords = unwords . map reverse . words

return () means it return an IO ()
return will not stop program

main = do
    a <- return "hell"
    b <- return "yeah!"
    putStrLn $ a ++ " " ++ b

Some IO methods

putStr :: String -> IO ()
putChar
putStrLn
print
- 接受任何是 Show typeclass 的 instance 的型态的值
- putStrLn 会打印出”, print不会
getChar :: IO Char

When

import Control.Monad will import when

import Control.Monad
main = do
    c <- getChar
    when (c /= ' ') $ do
        putChar c
        main

sequence

sequence :: [IO a] -> IO [a]

main = do
    a <- getLine
    b <- getLine
    c <- getLine
    print [a,b,c]
main = do
    rs <- sequence [getLine, getLine, getLine]
    print rs

mapM mapM_ forM

由于对一个串列 map 一个回传 I/O action 的函数，然后再 sequence 他这个动作太常用了

mapM 接受一个函数跟一个串列，将对串列用函数 map 然后 sequence 结果
mapM_ 也作同样的事，只是他把运算的结果丢掉而已

ghci> mapM print [1,2,3]
1
2
3
[(),(),()]
ghci> mapM_ print [1,2,3]
1
2
3
import Control.Monad
main = do
    colors <- forM [1,2,3,4] (\a -> do
        putStrLn $ "Which color do you associate with the number " ++ show a ++ "?"
        color <- getLine
        return color)
    putStrLn "The colors that you associate with 1, 2, 3 and 4 are: "
    mapM putStrLn colors

forever

forever 接受一个 I/O action 并回传一个永远作同一件事的 I/O action
import Control.Monad

import Control.Monad
import Data.Char
main = forever $ do
    putStr "Give me some input: "
    l <- getLine
    putStrLn $ map toUpper l

Stream

getChar
getLine
getContents :: IO String

import Data.Char
main = do
    contents <- getContents
    putStr (map toUpper contents)
$ cat haiku.txt | ./capslocker

File

man = do
    handle <- openFile "test.txt" ReadMode
    contents <- hGetContents handle
    putStr contents
    hClose handle

openFile :: FilePath -> IOMode -> IO Handle
type FilePath = String
data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode

import System.IO
main = do
    withFile "girlfriend.txt" ReadMode (\handle -> do
            contents <- hGetContents handle
            putStr contents)

withFile :: FilePath -> IOMode -> (Handle -> IO a) -> IO a

import System.IO
import Data.Char
main = do
    contents <- readFile "girlfriend.txt"
    writeFile "girlfriendcaps.txt" (map toUpper contents)

import System.IO
import Data.Char
main = do
    contents <- readFile "girlfriend.txt"
    writeFile "girlfriendcaps.txt" (map toUpper contents)

writefile :: FilePath -> String -> IO ()

import System.IO
main = do
    todoItem <- getLine
    appendFile "todo.txt" (todoItem ++ "\n")

Other functions:

hGetLine
hPutStr
hPutStrLn
hGetChar

Random

ghci> randomR (1,6) (mkStrGen 359353)
(6,1494289578 40692)
ghci> take 10 $ randomRs ('a','z') (mkStdGen 3) :: [Char]
"ndkxbvmomg"

System IO action

import System.Random
main = do
    gen <- getStdGen
    putStr $ take 20 (randomRs ('a','z') gen)
    gen' <- newStdGen
    putStr $ take 20 (randomRs ('a','z') gen')