The first step with any new technology is getting it running. The goal of this chapter is to get you started with a simple Yesod application, and cover some of the basic concepts and terminology.

Hello World

Let’s get this book started properly: a simple web page that says Hello World:

{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE QuasiQuotes           #-}
{-# LANGUAGE TemplateHaskell       #-}
{-# LANGUAGE TypeFamilies          #-}
import           Yesod

data HelloWorld = HelloWorld

mkYesod "HelloWorld" [parseRoutes|
/ HomeR GET

instance Yesod HelloWorld

getHomeR :: Handler Html
getHomeR = defaultLayout [whamlet|Hello World!|]

main :: IO ()
main = warp 3000 HelloWorld

If you save that code in helloworld.hs and run it with runhaskell helloworld.hs, you’ll get a web server running on port 3000. Note, if you followed the quick start guide and installed yesod with stack then you will not have runhaskell and will need to run stack runghc helloworld.hs instead. If you point your browser to http://localhost:3000, you’ll get the following HTML:

<!DOCTYPE html>
<html><head><title></title></head><body>Hello World!</body></html>

We’ll refer back to this example through the rest of the chapter.


Like most modern web frameworks, Yesod follows a front controller pattern. This means that every request to a Yesod application enters at the same point and is routed from there. As a contrast, in systems like PHP and ASP you usually create a number of different files, and the web server automatically directs requests to the relevant file.

In addition, Yesod uses a declarative style for specifying routes. In our example above, this looked like:

mkYesod "HelloWorld" [parseRoutes|
/ HomeR GET

In English, all this means is: In the HelloWorld application, create one route. I’d like to call it HomeR, it should listen for requests to / (the root of the application), and should answer GET requests. We call HomeR a resource, which is where the "R" suffix comes from.

The mkYesod TH function generates quite a bit of code here: a route data type, parser/render functions, a dispatch function, and some helper types. We’ll look at this in more detail in the routing chapter. But by using the -ddump-splices GHC option, we can get an immediate look at the generated code. A much cleaned up version of it is:

instance RenderRoute HelloWorld where
    data Route HelloWorld = HomeR
        deriving (Show, Eq, Read)
    renderRoute HomeR = ([], [])

instance ParseRoute HelloWorld where
    parseRoute ([], _) = Just HomeR
    parseRoute _       = Nothing

instance YesodDispatch HelloWorld where
    yesodDispatch env req =
        yesodRunner handler env mroute req
        mroute = parseRoute (pathInfo req, textQueryString req)
        handler =
            case mroute of
                Nothing -> notFound
                Just HomeR ->
                    case requestMethod req of
                        "GET" -> getHomeR
                        _     -> badMethod

type Handler = HandlerT HelloWorld IO

We can see that the RenderRoute class defines an associated data type providing the routes for our application. In this simple example, we have just one route: HomeR. In real life applications, we’ll have many more, and they will be more complicated than our HomeR.

renderRoute takes a route and turns it into path segments and query string parameters. Again, our example is simple, so the code is likewise simple: both values are empty lists.

ParseRoute provides the inverse function, parseRoute. Here we see the first strong motivation for our reliance on Template Haskell: it ensures that the parsing and rendering of routes correspond correctly with each other. This kind of code can easily become difficult to keep in sync when written by hand. By relying on code generation, we’re letting the compiler (and Yesod) handle those details for us.

YesodDispatch provides a means of taking an input request and passing it to the appropriate handler function. The process is essentially:

  1. Parse the request.

  2. Choose a handler function.

  3. Run the handler function.

The code generation follows a simple format for matching routes to handler function names, which we’ll describe in the next section.

Finally, we have a simple type synonym defining Handler to make our code a little easier to write.

There’s a lot more going on here than we’ve described. The generated dispatch code actually uses the view patterns language extension for efficiency, more type class instances are created, and there are other cases to handle such as subsites. We’ll get into the details as we go through the book, especially in the “Understanding a Request” chapter.

Handler function

So we have a route named HomeR, and it responds to GET requests. How do you define your response? You write a handler function. Yesod follows a standard naming scheme for these functions: it’s the lower case method name (e.g., GET becomes get) followed by the route name. In this case, the function name would be getHomeR.

Most of the code you write in Yesod lives in handler functions. This is where you process user input, perform database queries and create responses. In our simple example, we create a response using the defaultLayout function. This function wraps up the content it’s given in your site’s template. By default, it produces an HTML file with a doctype and html, head and body tags. As we’ll see in the Yesod typeclass chapter, this function can be overridden to do much more.

In our example, we pass [whamlet|Hello World!|] to defaultLayout. whamlet is another quasi-quoter. In this case, it converts Hamlet syntax into a Widget. Hamlet is the default HTML templating engine in Yesod. Together with its siblings Cassius, Lucius and Julius, you can create HTML, CSS and Javascript in a fully type-safe and compile-time-checked manner. We’ll see much more about this in the Shakespeare chapter.

Widgets are another cornerstone of Yesod. They allow you to create modular components of a site consisting of HTML, CSS and Javascript and reuse them throughout your site. We’ll get into more detail on them in the widgets chapter.

The Foundation

The word ‘HelloWorld’ shows up a number of times in our example. Every Yesod application has a foundation datatype. This datatype must be an instance of the Yesod typeclass, which provides a central place for declaring a number of different settings controlling the execution of our application.

In our case, this datatype is pretty boring: it doesn’t contain any information. Nonetheless, the foundation is central to how our example runs: it ties together the routes with the instance declaration and lets it all be run. We’ll see throughout this book that the foundation pops up in a whole bunch of places.

But foundations don’t have to be boring: they can be used to store lots of useful information, usually stuff that needs to be initialized at program launch and used throughout. Some very common examples are:

  • A database connection pool.

  • Settings loaded from a config file.

  • An HTTP connection manager.

  • A random number generator.


Once again we mention HelloWorld in our main function. Our foundation contains all the information we need to route and respond to requests in our application; now we just need to convert it into something that can run. A useful function for this in Yesod is warp, which runs the Warp webserver with a number of default settings enabled on the specified port (here, it’s 3000).

One of the features of Yesod is that you aren’t tied down to a single deployment strategy. Yesod is built on top of the Web Application Interface (WAI), allowing it to run on FastCGI, SCGI, Warp, or even as a desktop application using the Webkit library. We’ll discuss some of these options in the deployment chapter. And at the end of this chapter, we will explain the development server.

Warp is the premiere deployment option for Yesod. It is a lightweight, highly efficient web server developed specifically for hosting Yesod. It is also used outside of Yesod for other Haskell development (both framework and non-framework applications), as well as a standard file server in a number of production environments.

Resources and type-safe URLs

In our hello world, we defined just a single resource (HomeR). A web application is usually much more exciting with more than one page on it. Let’s take a look:

{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE QuasiQuotes           #-}
{-# LANGUAGE TemplateHaskell       #-}
{-# LANGUAGE TypeFamilies          #-}
import           Yesod

data Links = Links

mkYesod "Links" [parseRoutes|
/ HomeR GET
/page1 Page1R GET
/page2 Page2R GET

instance Yesod Links

getHomeR  = defaultLayout [whamlet|<a href=@{Page1R}>Go to page 1!|]
getPage1R = defaultLayout [whamlet|<a href=@{Page2R}>Go to page 2!|]
getPage2R = defaultLayout [whamlet|<a href=@{HomeR}>Go home!|]

main = warp 3000 Links

Overall, this is very similar to Hello World. Our foundation is now Links instead of HelloWorld, and in addition to the HomeR resource, we’ve added Page1R and Page2R. As such, we’ve also added two more handler functions: getPage1R and getPage2R.

The only truly new feature is inside the whamlet quasi-quotation. We’ll delve into syntax in the “Shakespeare” chapter, but we can see that:

<a href=@{Page1R}>Go to page 1!

creates a link to the Page1R resource. The important thing to note here is that Page1R is a data constructor. By making each resource a data constructor, we have a feature called type-safe URLs. Instead of splicing together strings to create URLs, we simply create a plain old Haskell value. By using at-sign interpolation (@{…}), Yesod automatically renders those values to textual URLs before sending things off to the user. We can see how this is implemented by looking again at the -ddump-splices output:

instance RenderRoute Links where
    data Route Links = HomeR | Page1R | Page2R
      deriving (Show, Eq, Read)

    renderRoute HomeR  = ([], [])
    renderRoute Page1R = (["page1"], [])
    renderRoute Page2R = (["page2"], [])

In the Route associated type for Links, we have additional constructors for Page1R and Page2R. We also now have a better glimpse of the return values for renderRoute. The first part of the tuple gives the path pieces for the given route. The second part gives the query string parameters; for almost all use cases, this will be an empty list.

It’s hard to over-estimate the value of type-safe URLs. They give you a huge amount of flexibility and robustness when developing your application. You can move URLs around at will without ever breaking links. In the routing chapter, we’ll see that routes can take parameters, such as a blog entry URL taking the blog post ID.

Let’s say you want to switch from routing on the numerical post ID to a year/month/slug setup. In a traditional web framework, you would need to go through every single reference to your blog post route and update appropriately. If you miss one, you’ll have 404s at runtime. In Yesod, all you do is update your route and compile: GHC will pinpoint every single line of code that needs to be corrected.

Non-HTML responses

Yesod can serve up any kind of content you want, and has first-class support for many commonly used response formats. You’ve seen HTML so far, but JSON data is just as easy, via the aeson package:

{-# LANGUAGE ExtendedDefaultRules #-}
{-# LANGUAGE OverloadedStrings    #-}
{-# LANGUAGE QuasiQuotes          #-}
{-# LANGUAGE TemplateHaskell      #-}
{-# LANGUAGE TypeFamilies         #-}
import Yesod

data App = App

mkYesod "App" [parseRoutes|
/ HomeR GET

instance Yesod App

getHomeR  = return $ object ["msg" .= "Hello World"]

main = warp 3000 App

We’ll cover JSON responses in more detail in later chapters, including how to automatically switch between HTML and JSON representations depending on the Accept request header.

The scaffolded site

We’ll use stack to install the Yesod libraries and the yesod helper executable. Then we’ll use stack new to create a scaffolded site. It will generate a folder containing the default scaffolded site. Inside that folder, you can run stack install --only-dependencies to build any extra dependencies (such as your database backends), and then yesod devel to run your site.

The scaffolded site gives you a lot of best practices out of the box, setting up files and dependencies in a time-tested approach used by most production Yesod sites. However, all this convenience can get in the way of actually learning Yesod. Therefore, most of this book will avoid the scaffolding tool, and instead deal directly with Yesod as a library. But if you’re going to build a real site, I strongly recommend using the scaffolding.

We will cover the structure of the scaffolded site in the scaffolding chapter.

Development server

One of the advantages interpreted languages have over compiled languages is fast prototyping: you save changes to a file and hit refresh. If we want to make any changes to our Yesod apps above, we’ll need to call runhaskell from scratch, which can be a bit tedious.

Fortunately, there’s a solution to this: yesod devel automatically rebuilds and reloads your code for you. This can be a great way to develop your Yesod projects, and when you’re ready to move to production, you still get to compile down to incredibly efficient code. The Yesod scaffolding automatically sets things up for you. This gives you the best of both worlds: rapid prototyping and fast production code.

It’s a little bit more involved to set up your code to be used by yesod devel, so our examples will just use warp. Fortunately, the scaffolded site is fully configured to use the development server, so when you’re ready to move over to the real world, it will be waiting for you.


Every Yesod application is built around a foundation datatype. We associate some resources with that datatype and define some handler functions, and Yesod handles all of the routing. These resources are also data constructors, which lets us have type-safe URLs.

By being built on top of WAI, Yesod applications can run with a number of different backends. For simple apps, the warp function provides a convenient way to use the Warp web server. For rapid development, using yesod devel is a good choice. And when you’re ready to move to production, you have the full power and flexibility to configure Warp (or any other WAI handler) to suit your needs.

When developing in Yesod, we get a number of choices for coding style: quasi-quotation or external files, warp or yesod devel, and so on. The examples in this book will tend towards using the choices that are easiest to copy-and-paste, but the more powerful options will be available when you start building real Yesod applications.