**Course Title:** Functional Programming with Haskell: From Fundamentals to Advanced Concepts **Section Title:** Modules and Code Organization in Haskell **Topic:** Build a small project by splitting code into multiple modules.(Lab topic) **Objective:** In this lab topic, we will apply our knowledge of Haskell modules to build a small project by splitting the code into multiple modules. This exercise will help reinforce our understanding of how to organize and structure larger Haskell projects effectively. **Prerequisites:** Before starting this lab topic, make sure you have completed the following topics: * Understanding Haskell modules and importing libraries * Creating and using custom modules in Haskell * Managing dependencies with Cabal and Stack **Step 1: Define the Project Structure** We will build a simple command-line calculator that takes in mathematical expressions and evaluates them. Our project structure will consist of the following modules: * `Main.hs`: The entry point of our application * `Calculator.hs`: Contains the calculator logic * `Parser.hs`: Responsible for parsing the input expressions * `Utilities.hs`: Provides utility functions for the calculator Create a new directory for your project and initialize it with Cabal: ```bash mkdir calculator cd calculator cabal init ``` **Step 2: Create the `Utilities.hs` Module** In this module, we will define a function to calculate the greatest common divisor (GCD) of two numbers. ```haskell -- Utilities.hs module Utilities where gcd :: Int -> Int -> Int gcd a b | b == 0 = a | otherwise = gcd b (a `mod` b) ``` **Step 3: Create the `Parser.hs` Module** In this module, we will define a function to parse the input expressions. ```haskell -- Parser.hs module Parser where import Text.Parsec import Text.Parsec.String parseExpression :: String -> Either ParseError Int parseExpression input = parse expression "" input expression :: Parser Int expression = do n <- many1 digit return $ read n ``` We will use the `parsec` library to define our parser. Make sure to add it as a dependency in your `calculator.cabal` file: ```cabal build-depends: base >= 4.7 && < 5 , parsec >= 3.1 && < 3.2 ``` **Step 4: Create the `Calculator.hs` Module** In this module, we will define the calculator logic. ```haskell -- Calculator.hs module Calculator where import Utilities import Parser calculate :: String -> Either ParseError Int calculate input = do parsedInput <- parseExpression input return $ gcd parsedInput 10 ``` **Step 5: Create the `Main.hs` Module** In this module, we will define the entry point of our application. ```haskell -- Main.hs module Main where import Calculator main :: IO () main = do putStrLn "Enter a mathematical expression:" input <- getLine case calculate input of Right result -> putStrLn $ "Result: " ++ show result Left err -> putStrLn $ "Error: " ++ show err ``` **Step 6: Build and Run the Project** Build the project using Cabal: ```bash cabal build ``` Run the project: ```bash cabal run ``` This should start the calculator application, and you should be able to enter mathematical expressions to evaluate them. **Conclusion:** In this lab topic, we have successfully built a small command-line calculator by splitting the code into multiple modules. We have applied our knowledge of Haskell modules to organize and structure the project effectively. **Takeaways:** * Haskell modules are a powerful tool for organizing and structuring larger projects. * Breaking down the code into smaller modules makes it easier to maintain and understand. * Using Cabal to manage dependencies and build the project simplifies the development process. **Further Reading:** * [Haskell Wiki: Modules](https://wiki.haskell.org/Module) * [Haskell Wiki: Cabal](https://wiki.haskell.org/Cabal) * [Parsec Documentation](https://hackage.haskell.org/package/parsec-3.1.14.0/docs/Text-Parsec.html) **Exercise:** * Add more functionality to the calculator, such as supporting multiple operators or parsing more complex expressions. * Experiment with different parsing libraries, such as `attoparsec` or `parsec-combinators`. **Leave a comment or ask for help:** If you have any questions or need help with this lab topic, please leave a comment below.