Haskell IO — Interacting with the World

main :: IO ()
main = do
    putStrLn "What is your name?"
    name <- getLine
    let greeting = "Hello, " ++ name ++ "!"
    putStrLn greeting

putStrLn "Hello!"           -- print with newline
putStr "No newline"         -- print without newline
putChar 'A'                 -- print one character

name <- getLine             -- read a line from stdin
-- ⚠️ getLine is unsafe — use readMaybe for numbers

import System.IO
import System.Directory

-- Read entire file
contents <- readFile "data.txt"
putStrLn contents

-- Write file (overwrites)
writeFile "output.txt" "Hello, Nepal!\n"

-- Append to file
appendFile "log.txt" "New entry\n"

-- With handle (more control)
handle <- openFile "data.txt" ReadMode
contents <- hGetContents handle
putStrLn contents
hClose handle

import Text.Read (readMaybe)

main :: IO ()
main = do
    putStr "Enter your age: "
    input <- getLine
    case readMaybe input :: Maybe Int of
        Just age -> putStrLn $ "You are " ++ show age ++ " years old."
        Nothing  -> putStrLn "That's not a valid number."

-- IO is a type — functions returning IO a are actions, not values
greet :: IO ()
greet = putStrLn "Hello!"

-- do notation desugars to >>= (bind)
greet = putStrLn "Hello!" >> putStrLn "World!"

-- Or with explicit bind
greet = putStrLn "Hello!" >>= \_ -> putStrLn "World!"