**Course Title:** Functional Programming with Haskell: From Fundamentals to Advanced Concepts **Section Title:** Monads and Functors in Haskell **Topic:** Use monads to build a simple Haskell program that handles IO and errors using `Maybe` or `Either` (Lab topic) ### Overview In this lab, you will learn how to use monads to build a simple Haskell program that handles IO and errors using `Maybe` or `Either`. You will apply your knowledge of monads, `Maybe`, and `Either` to write a program that interacts with the user, performs calculations, and handles potential errors. ### Learning Objectives * Understand how to use monads to handle IO and errors in Haskell * Learn how to use `Maybe` and `Either` to handle errors in a monadic context * Practice using `do` notation and chaining operations with `>>=` to write concise and readable code ### Using Monads to Handle IO and Errors In Haskell, monads provide a way to handle effects in a pure functional programming context. The `IO` monad is used to handle input/output operations, while `Maybe` and `Either` are used to handle errors. #### Using `Maybe` to Handle Errors The `Maybe` monad is a simple way to handle errors in a monadic context. It represents a computation that may fail, returning `Nothing` in case of an error. ```haskell import Control.Monad (liftM2) -- Define a function that divides two numbers and returns a Maybe result divide :: Int -> Int -> Maybe Int divide _ 0 = Nothing divide x y = Just (x `div` y) -- Use do notation to write a concise and readable program calculation :: Int -> Int -> Maybe Int calculation x y = do result <- divide x y return result -- Use the calculation function in a monadic context main :: IO () main = do putStrLn "Enter two numbers:" x <- readLn y <- readLn case calculation x y of Just result -> putStrLn ("Result: " ++ show result) Nothing -> putStrLn "Error: Division by zero!" ``` #### Using `Either` to Handle Errors The `Either` monad is a more powerful way to handle errors in a monadic context. It represents a computation that may fail, returning an error message or a successful result. ```haskell import Control.Monad (liftM2) -- Define a function that divides two numbers and returns an Either result divide :: Int -> Int -> Either String Int divide _ 0 = Left "Error: Division by zero!" divide x y = Right (x `div` y) -- Use do notation to write a concise and readable program calculation :: Int -> Int -> Either String Int calculation x y = do result <- divide x y return result -- Use the calculation function in a monadic context main :: IO () main = do putStrLn "Enter two numbers:" x <- readLn y <- readLn case calculation x y of Right result -> putStrLn ("Result: " ++ show result) Left err -> putStrLn err ``` ### Exercises 1. Modify the `calculation` function to handle multiple errors using `Either`. 2. Use the `IO` monad to read a file and handle errors using `Maybe` or `Either`. 3. Write a program that uses monads to handle IO and errors in a real-world scenario. ### Resources * [Haskell documentation: `Maybe`](https://hackage.haskell.org/package/base-4.16.0.0/docs/Data-Maybe.html) * [Haskell documentation: `Either`](https://hackage.haskell.org/package/base-4.16.0.0/docs/Data-Either.html) * [Haskell documentation: `IO`](https://hackage.haskell.org/package/base-4.16.0.0/docs/System-IO.html) ### Conclusion In this lab, you learned how to use monads to handle IO and errors in Haskell. You applied your knowledge of `Maybe` and `Either` to write a program that interacts with the user, performs calculations, and handles potential errors. You also practiced using `do` notation and chaining operations with `>>=` to write concise and readable code. ### Leave a Comment/Ask for Help If you have any questions or need help with the exercises, please leave a comment below. We'll be happy to help you understand the material better. Next, we'll be covering **Understanding Haskell's approach to side effects and IO** in **Input/Output and Working with Side Effects**.