Get Programming with Haskell

Lesson 21. Hello World!—introducing IO types

After reading lesson 21, you’ll be able to

• Understand how Haskell handles I/O by using IO types
• Use do-notation to perform I/O
• Write pure programs that interact with the real world

In lesson 1, you saw a basic example of a Hello World program. In this lesson, you’ll revisit a similar program to get a better sense of how I/O works in Haskell. Here’s an example program using I/O that reads a name from the command line and prints out "Hello <name>!".

Listing 21.1. A simple Hello World program

helloPerson :: String -> String helloPerson name = "Hello" ++ " " ++ name ++ "!" main :: IO () main = do putStrLn "Hello! What's your name?" name <- getLine let statement = helloPerson name putStrLn statement ...