IO基础 #

-- IO是一个类型构造器
-- IO a 表示一个产生类型a值的IO操作

-- 示例
getLine :: IO String          -- 读取一行
putStrLn :: String -> IO ()   -- 输出一行
getChar :: IO Char            -- 读取一个字符

-- 纯函数：相同输入总是产生相同输出
add :: Int -> Int -> Int
add x y = x + y

-- IO操作：有副作用
main :: IO ()
main = do
    putStrLn "Enter your name:"
    name <- getLine
    putStrLn ("Hello, " ++ name ++ "!")

-- putStrLn：输出字符串并换行
putStrLn :: String -> IO ()

-- putStr：输出字符串不换行
putStr :: String -> IO ()

-- putChar：输出单个字符
putChar :: Char -> IO ()

-- print：输出任意Show类型
print :: Show a => a -> IO ()

-- 示例
main :: IO ()
main = do
    putStrLn "Hello, World!"
    putStr "No newline"
    putChar '!'
    print 42

-- getLine：读取一行
getLine :: IO String

-- getChar：读取一个字符
getChar :: IO Char

-- getContents：读取全部内容（惰性）
getContents :: IO String

-- 示例
main :: IO ()
main = do
    putStrLn "Enter your name:"
    name <- getLine
    putStrLn ("Hello, " ++ name ++ "!")

-- interact：将函数应用于整个输入
interact :: (String -> String) -> IO ()

-- 示例：将输入转为大写
main :: IO ()
main = interact (map toUpper)

-- 示例：只输出包含特定字符串的行
main :: IO ()
main = interact (unlines . filter (elem 'a') . lines)

-- 使用>>顺序执行
(>>) :: IO a -> IO b -> IO b

-- 示例
main :: IO ()
main = 
    putStrLn "First" >>
    putStrLn "Second" >>
    putStrLn "Third"

-- 使用>>=绑定IO结果
(>>=) :: IO a -> (a -> IO b) -> IO b

-- 示例
main :: IO ()
main = 
    getLine >>= \name ->
    putStrLn ("Hello, " ++ name)

-- return：将值包装为IO
return :: a -> IO a

-- 示例
main :: IO ()
main = do
    x <- return 5
    print x  -- 5

-- 注意：return不是返回语句
-- 它只是将值包装为IO

-- 好：纯函数与IO分离
processName :: String -> String
processName name = "Hello, " ++ name ++ "!"

main :: IO ()
main = do
    name <- getLine
    putStrLn (processName name)

-- 不好：在纯函数中做IO
-- processName name = unsafePerformIO (putStrLn "debug")

-- 不能直接从IO提取值
-- 错误示例：
-- name :: String
-- name = getLine  -- 类型错误！

-- 正确方式：在IO上下文中使用
main :: IO ()
main = do
    name <- getLine  -- name :: String
    putStrLn name

-- IO是Functor
-- 可以使用fmap处理IO结果

-- 示例
main :: IO ()
main = do
    -- fmap处理IO结果
    upperName <- fmap (map toUpper) getLine
    putStrLn upperName

    -- 等价于
    name <- getLine
    let upperName' = map toUpper name
    putStrLn upperName'

import Control.Monad

-- sequence：将IO列表转为IO的列表
sequence :: [IO a] -> IO [a]

-- 示例
main :: IO ()
main = do
    results <- sequence [getLine, getLine, getLine]
    print results

-- sequence_：丢弃结果
sequence_ :: [IO a] -> IO ()

main' :: IO ()
main' = sequence_ [putStrLn "One", putStrLn "Two", putStrLn "Three"]

import Control.Monad

-- mapM：映射后序列
mapM :: (a -> IO b) -> [a] -> IO [b]

-- 示例
main :: IO ()
main = do
    results <- mapM getLine ["First: ", "Second: ", "Third: "]
    print results

-- mapM_：丢弃结果
mapM_ :: (a -> IO b) -> [a] -> IO ()

main' :: IO ()
main' = mapM_ putStrLn ["One", "Two", "Three"]

import Control.Monad

-- forM：mapM的参数翻转
forM :: [a] -> (a -> IO b) -> IO [b]

-- 示例
main :: IO ()
main = do
    results <- forM [1..3] $ \i -> do
        putStrLn $ "Enter name " ++ show i ++ ":"
        getLine
    print results

import Control.Monad

-- when：条件为True时执行
when :: Bool -> IO () -> IO ()

-- unless：条件为False时执行
unless :: Bool -> IO () -> IO ()

-- 示例
main :: IO ()
main = do
    putStrLn "Enter a number:"
    n <- readLn
    when (n > 0) $ putStrLn "Positive!"
    when (n < 0) $ putStrLn "Negative!"
    when (n == 0) $ putStrLn "Zero!"

main :: IO ()
main = do
    putStrLn "Simple Calculator"
    putStrLn "Enter first number:"
    a <- readLn :: IO Int
    putStrLn "Enter second number:"
    b <- readLn :: IO Int
    putStrLn "Enter operation (+, -, *, /):"
    op <- getLine
    case op of
        "+" -> print (a + b)
        "-" -> print (a - b)
        "*" -> print (a * b)
        "/" -> print (a `div` b)
        _   -> putStrLn "Unknown operation"

import System.Random

main :: IO ()
main = do
    target <- randomRIO (1, 100) :: IO Int
    putStrLn "I'm thinking of a number between 1 and 100"
    gameLoop target

gameLoop :: Int -> IO ()
gameLoop target = do
    putStrLn "Enter your guess:"
    guess <- readLn :: IO Int
    if guess < target
        then do
            putStrLn "Too low!"
            gameLoop target
        else if guess > target
            then do
                putStrLn "Too high!"
                gameLoop target
            else putStrLn "Correct! You win!"

main :: IO ()
main = do
    putStrLn "Enter source file:"
    src <- getLine
    putStrLn "Enter destination file:"
    dst <- getLine
    contents <- readFile src
    writeFile dst contents
    putStrLn "File copied successfully!"