**Course Title:** Functional Programming with Haskell: From Fundamentals to Advanced Concepts **Section Title:** Web Development in Haskell **Topic:** Build a simple web application using a Haskell web framework such as Yesod or Servant.(Lab topic) **Overview** In this lab, we'll guide you through building a simple web application using the Yesod web framework. You'll learn how to create a basic web application, define routes, handle user input, and render templates. **Prerequisites** * Familiarity with Haskell fundamentals * Understanding of Yesod or Servant web framework concepts * Installation of the Yesod or Servant package using Cabal or Stack * Basic knowledge of HTML and CSS **Step 1: Setting up the Project** To create a new Yesod project, run the following command in your terminal: ```bash yesod init ``` This will prompt you to choose a project name and location. Choose a name for your project, and Yesod will create a new directory with the necessary files and dependencies. **Step 2: Defining Routes** In Yesod, routes are defined in the `config/routes` file. Open this file and add the following route: ```haskell / HomeR GET ``` This defines a new route for the root URL (`/`) that responds to GET requests and maps to the `HomeR` handler. **Step 3: Creating Handlers** In Yesod, handlers are functions that respond to requests and return a response. Create a new file called `Handler/Home.hs` and add the following code: ```haskell module Handler.Home where import Yesod getHomeR :: Handler Html getHomeR = defaultLayout $ do setTitle "My Yesod App" [whamlet| <h1>Welcome to my Yesod app! |] ``` This defines a new handler for the `HomeR` route that returns an HTML response with a title and a heading. **Step 4: Rendering Templates** Yesod uses a templating system called Hamlet to render HTML templates. Create a new file called `templates/home.hamlet` and add the following code: ```hamlet <h1>Welcome to my Yesod app! ``` This defines a simple HTML template that we'll use to render the home page. **Step 5: Running the Application** To run the application, navigate to the project directory and run the following command: ```bash yesod devel ``` This will start the development server, and you can access your application by visiting `http://localhost:3000` in your web browser. **Example Use Cases** * Creating a blog with Yesod * Building a RESTful API with Yesod * Creating a web application with user authentication using Yesod **External Resources** * Yesod documentation: <https://www.yesodweb.com/> * Servant documentation: <https://haskell-servant.github.io/> **Practical Takeaways** * Understand how to define routes and handlers in Yesod * Learn how to render templates using Hamlet * Understand how to create a basic web application with Yesod **Exercise** * Create a new route that responds to POST requests and handles user input * Create a new handler that stores user input in a database * Create a new template that displays user data **Leave a Comment or Ask for Help** If you have any questions or need help with this topic, please leave a comment below. We'd be happy to assist you. **Next Topic** In the next topic, we'll cover packaging and distributing Haskell applications. We'll discuss the different options for deploying Haskell applications and how to create a package for distribution.