Routing and Handlers

If we look at Yesod as a Model-View-Controller framework, routing and handlers make up the controller. For contrast, let’s describe two other routing approaches used in other web development environments:

  • Dispatch based on file name. This is how PHP and ASP work, for example.

  • Have a centralized routing function that parses routes based on regular expressions. Django and Rails follow this approach.

Yesod is closer in principle to the latter technique. Even so, there are significant differences. Instead of using regular expressions, Yesod matches on pieces of a route. Instead of having a one-way route-to-handler mapping, Yesod has an intermediate data type (called the route datatype, or a type-safe URL) and creates two-way conversion functions.

Coding this more advanced system manually is tedious and error prone. Therefore, Yesod defines a Domain Specific Language (DSL) for specifying routes, and provides Template Haskell functions to convert this DSL to Haskell code. This chapter will explain the syntax of the routing declarations, give you a glimpse of what code is generated for you, and explain the interaction between routing and handler functions.

Route Syntax

Instead of trying to shoe-horn route declarations into an existing syntax, Yesod’s approach is to use a simplified syntax designed just for routes. This has the advantage of making the code not only easy to write, but simple enough for someone with no Yesod experience to read and understand the sitemap of your application.

A basic example of this syntax is:

/             HomeR     GET
/blog         BlogR     GET POST
/blog/#BlogId BlogPostR GET POST

/static       StaticR   Static getStatic

The next few sections will explain the full details of what goes on in the route declaration.


One of the first things Yesod does when it gets a request is split up the requested path into pieces. The pieces are tokenized at all forward slashes. For example:

toPieces "/" = []
toPieces "/foo/bar/baz/" = ["foo", "bar", "baz", ""]

You may notice that there are some funny things going on with trailing slashes, or double slashes ("/foo//bar//"), or a few other things. Yesod believes in having canonical URLs; if users request a URL with a trailing slash, or with a double slash, they are automatically redirected to the canonical version. This ensures you have one URL for one resource, and can help with your search rankings.

What this means for you is that you needn’t concern yourself with the exact structure of your URLs: you can safely think about pieces of a path, and Yesod automatically handles intercalating the slashes and escaping problematic characters.

If, by the way, you want more fine-tuned control of how paths are split into pieces and joined together again, you’ll want to look at the cleanPath and joinPath methods in the Yesod typeclass chapter.

Types of Pieces

When you are declaring your routes, you have three types of pieces at your disposal:


This is a plain string that must be matched against precisely in the URL.

Dynamic single

This is a single piece (ie, between two forward slashes), but represents a user-submitted value. This is the primary method of receiving extra user input on a page request. These pieces begin with a hash (#) and are followed by a data type. The datatype must be an instance of PathPiece.

Dynamic multi

The same as before, but can receive multiple pieces of the URL. This must always be the last piece in a resource pattern. It is specified by an asterisk (*) followed by a datatype, which must be an instance of PathMultiPiece. Multi pieces are not as common as the other two, though they are very important for implementing features like static trees representing file structure or wikis with arbitrary hierarchies.

Let us take a look at some standard kinds of resource patterns you may want to write. Starting simply, the root of an application will just be /. Similarly, you may want to place your FAQ at /page/faq.

Now let’s say you are going to write a Fibonacci website. You may construct your URLs like /fib/#Int. But there’s a slight problem with this: we do not want to allow negative numbers or zero to be passed into our application. Fortunately, the type system can protect us:

newtype Natural = Natural Int
    deriving (Eq, Show, Read)

instance PathPiece Natural where
    toPathPiece (Natural i) = T.pack $ show i
    fromPathPiece s =
        case reads $ T.unpack s of
            (i, ""):_
                | i < 1 -> Nothing
                | otherwise -> Just $ Natural i
            [] -> Nothing

On line 1 we define a simple newtype wrapper around Int to protect ourselves from invalid input. We can see that PathPiece is a typeclass with two methods. toPathPiece does nothing more than convert to a Text. fromPathPiece attempts to convert a Text to our datatype, returning Nothing when this conversion is impossible. By using this datatype, we can ensure that our handler function is only ever given natural numbers, allowing us to once again use the type system to battle the boundary issue.

Defining a PathMultiPiece is just as simple. Let’s say we want to have a Wiki with at least two levels of hierarchy; we might define a datatype such as:

data Page = Page Text Text [Text] -- 2 or more
    deriving (Eq, Show, Read)

instance PathMultiPiece Page where
    toPathMultiPiece (Page x y z) = x : y : z
    fromPathMultiPiece (x:y:z) = Just $ Page x y z
    fromPathMultiPiece _ = Nothing

Overlap checking

By default, Yesod will ensure that no two routes have the potential to overlap with each other. So, for example, consider the following routes:

/foo/bar   Foo1R GET
/foo/#Text Foo2R GET

This route declaration will be rejected as overlapping, since /foo/bar will match both routes. However, there are two cases where we may wish to allow overlapping:

  1. We know by the definition of our datatype that the overlap can never happen. For example, if you replace Text with Int above, it’s easy to convince yourself that there’s no route that exists that will overlap. Yesod is currently not capable of performing such an analysis.

  2. You have some extra knowledge about how your application operates, and know that such a situation should never be allowed. For example, if the Foo2R route should never be allowed to receive the parameter bar.

You can turn off overlap checking by using the exclamation mark at the beginning of your route. For example, the following will be accepted by Yesod:

/foo/bar    Foo1R GET
!/foo/#Int  Foo2R GET
!/foo/#Text Foo3R GET

One issue that overlapping routes introduce is ambiguity. In the example above, should /foo/bar route to Foo1R or Foo3R? And should /foo/42 route to Foo2R or Foo3R? Yesod’s rule for this is simple: first route wins.

Empty #String or #Text as dynamic piece

Consider the following route declaration:

/hello          HelloR     GET
/hello/#String  HelloNameR GET

Let’s say a user requests the path /hello/ – which handler would respond to the request?

It will be HelloR because Yesod’s dispatch mechanism removes trailing slashes and redirects to the canonical form of the URL.

If users actually want to address the HelloNameR handler with an empty string as argument they need to request the path /hello/- instead. Yesod automatically converts the Minus sign to the empty string.

Likewise, the resulting URL for @{HelloNameR ""} would be /hello/-.

Also, to disambiguate a single actual -, Yesod prefixes that piece with another Minus sign when rendering the URL. Consequently, Yesod also prefixes any string consisting only of Minus signs with one single Minus sign.

Resource name

Each resource pattern also has a name associated with it. That name will become the constructor for the type safe URL datatype associated with your application. Therefore, it has to start with a capital letter. By convention, these resource names all end with a capital R. There is nothing forcing you to do this, it is just common practice.

The exact definition of our constructor depends upon the resource pattern it is attached to. Whatever datatypes are used as single-pieces or multi-pieces of the pattern become arguments to the datatype. This gives us a 1-to-1 correspondence between our type-safe URL values and valid URLs in our application.

Let’s get some real examples going here. If you had the resource patterns /person/#Text named PersonR, /year/#Int named YearR and /page/faq named FaqR, you would end up with a route data type roughly looking like:

data MyRoute = PersonR Text
             | YearR Int
             | FaqR

If a user requests /year/2009, Yesod will convert it into the value YearR 2009. /person/Michael becomes PersonR "Michael" and /page/faq becomes FaqR. On the other hand, /year/two-thousand-nine, /person/michael/snoyman and /page/FAQ would all result in 404 errors without ever seeing your code.

Handler specification

The last piece of the puzzle when declaring your resources is how they will be handled. There are three options in Yesod:

  • A single handler function for all request methods on a given route.

  • A separate handler function for each request method on a given route. Any other request method will generate a 405 Method Not Allowed response.

  • You want to pass off to a subsite.

The first two can be easily specified. A single handler function will be a line with just a resource pattern and the resource name, such as /page/faq FaqR. In this case, the handler function must be named handleFaqR.

A separate handler for each request method will be the same, plus a list of request methods. The request methods must be all capital letters. For example, /person/#String PersonR GET POST DELETE. In this case, you would need to define three handler functions: getPersonR, postPersonR and deletePersonR.

Subsites are a very useful— but more complicated— topic in Yesod. We will cover writing subsites later, but using them is not too difficult. The most commonly used subsite is the static subsite, which serves static files for your application. In order to serve static files from /static, you would need a resource line like:

/static StaticR Static getStatic

In this line, /static just says where in your URL structure to serve the static files from. There is nothing magical about the word static, you could easily replace it with /my/non-dynamic/files.

The next word, StaticR, gives the resource name. The next two words specify that we are using a subsite. Static is the name of the subsite foundation datatype, and getStatic is a function that gets a Static value from a value of your master foundation datatype.

Let’s not get too caught up in the details of subsites now. We will look more closely at the static subsite in the scaffolded site chapter.


Once you have specified your routes, Yesod will take care of all the pesky details of URL dispatch for you. You just need to make sure to provide the appropriate handler functions. For subsite routes, you do not need to write any handler functions, but you do for the other two. We mentioned the naming rules above (MyHandlerR GET becomes getMyHandlerR, MyOtherHandlerR becomes handleMyOtherHandlerR).

Now that we know which functions we need to write, let’s figure out what their type signatures should be.

Return Type

Let’s look at a simple handler function:

mkYesod "Simple" [parseRoutes|
/ HomeR GET

getHomeR :: Handler Html
getHomeR = defaultLayout [whamlet|<h1>This is simple|]

There are two components to this return type: Handler and Html. Let’s analyze each in more depth.

Handler monad

Like the Widget type, the Handler data type is not defined anywhere in the Yesod libraries. Instead, the libraries provide the data type:

data HandlerT site m a

And like WidgetT, this has three arguments: a base monad m, a monadic value a, and the foundation data type site. Each application defines a Handler synonym which constrains site to that application’s foundation data type, and sets m to IO. If your foundation is MyApp, in other words, you’d have the synonym:

type Handler = HandlerT MyApp IO

We need to be able to modify the underlying monad when writing subsites, but otherwise we’ll use IO.

The HandlerT monad provides access to information about the user request (e.g. query-string parameters), allows modifying the response (e.g., response headers), and more. This is the monad that most of your Yesod code will live in.

In addition, there’s a type class called MonadHandler. Both HandlerT and WidgetT are instances of this type class, allowing many common functions to be used in both monads. If you see MonadHandler in any API documentation, you should remember that the function can be used in your Handler functions.


There’s nothing too surprising about this type. This function returns some HTML content, represented by the Html data type. But clearly Yesod would not be useful if it only allowed HTML responses to be generated. We want to respond with CSS, Javascript, JSON, images, and more. So the question is: what data types can be returned?

In order to generate a response, we need to know two pieces of information: the content type (e.g., text/html, image/png) and how to serialize it to a stream of bytes. This is represented by the TypedContent data type:

data TypedContent = TypedContent !ContentType !Content

We also have a type class for all data types which can be converted to a TypedContent:

class ToTypedContent a where
    toTypedContent :: a -> TypedContent

Many common data types are instances of this type class, including Html, Value (from the aeson package, representing JSON), Text, and even () (for representing an empty response).


Let’s return to our simple example from above:

mkYesod "Simple" [parseRoutes|
/ HomeR GET

getHomeR :: Handler Html
getHomeR = defaultLayout [whamlet|<h1>This is simple|]

Not every route is as simple as this HomeR. Take for instance our PersonR route from earlier. The name of the person needs to be passed to the handler function. This translation is very straight-forward, and hopefully intuitive. For example:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes       #-}
{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE TypeFamilies      #-}
{-# LANGUAGE ViewPatterns      #-}
import           Data.Text (Text)
import qualified Data.Text as T
import           Yesod

data App = App

mkYesod "App" [parseRoutes|
/person/#Text PersonR GET
/year/#Integer/month/#Text/day/#Int DateR
/wiki/*Texts WikiR GET

instance Yesod App

getPersonR :: Text -> Handler Html
getPersonR name = defaultLayout [whamlet|<h1>Hello #{name}!|]

handleDateR :: Integer -> Text -> Int -> Handler Text -- text/plain
handleDateR year month day =
    return $
        T.concat [month, " ", T.pack $ show day, ", ", T.pack $ show year]

getWikiR :: [Text] -> Handler Text
getWikiR = return . T.unwords

main :: IO ()
main = warp 3000 App

The arguments have the types of the dynamic pieces for each route, in the order specified. Also, notice how we are able to use both Html and Text return values.

The Handler functions

Since the majority of your code will live in the Handler monad, it’s important to invest some time in understanding it better. The remainder of this chapter will give a brief introduction to some of the most common functions living in the Handler monad. I am specifically not covering any of the session functions; that will be addressed in the sessions chapter.

Application Information

There are a number of functions that return information about your application as a whole, and give no information about individual requests. Some of these are:


Returns your application foundation value. If you store configuration values in your foundation, you will probably end up using this function a lot. (If you’re so inclined, you can also use ask from Control.Monad.Reader; getYesod is simply a type-constrained synonym for it.)


Returns the URL rendering function, which converts a type-safe URL into a Text. Most of the time- like with Hamlet- Yesod calls this function for you, but you may occasionally need to call it directly.


A variant of getUrlRender that converts both a type-safe URL and a list of query-string parameters. This function handles all percent-encoding necessary.

Request Information

The most common information you will want to get about the current request is the requested path, the query string parameters and POSTed form data. The first of those is dealt with in the routing, as described above. The other two are best dealt with using the forms module.

That said, you will sometimes need to get the data in a more raw format. For this purpose, Yesod exposes the YesodRequest datatype along with the getRequest function to retrieve it. This gives you access to the full list of GET parameters, cookies, and preferred languages. There are some convenient functions to make these lookups easier, such as lookupGetParam, lookupCookie and languages. For raw access to the POST parameters, you should use runRequestBody.

If you need even more raw data, like request headers, you can use waiRequest to access the Web Application Interface (WAI) request value. See the WAI appendix for more details.

Short Circuiting

The following functions immediately end execution of a handler function and return a result to the user.


Sends a redirect response to the user (a 303 response). If you want to use a different response code (e.g., a permanent 301 redirect), you can use redirectWith.


Return a 404 response. This can be useful if a user requests a database value that doesn’t exist.


Return a 403 response with a specific error message.


A 400 response with a list of invalid arguments.


Sends a file from the filesystem with a specified content type. This is the preferred way to send static files, since the underlying WAI handler may be able to optimize this to a sendfile system call. Using readFile for sending static files should not be necessary.


Send a normal response with a 200 status code. This is really just a convenience for when you need to break out of some deeply nested code with an immediate response. Any instance of ToTypedContent may be used.


When you need to get low-level and send out a raw WAI response. This can be especially useful for creating streaming responses or a technique like server-sent events.

Response Headers


Set a cookie on the client. Instead of taking an expiration date, this function takes a cookie duration in minutes. Remember, you won’t see this cookie using lookupCookie until the following request.


Tells the client to remove a cookie. Once again, lookupCookie will not reflect this change until the next request.


Set an arbitrary response header.


Set the preferred user language, which will show up in the result of the languages function.


Set a Cache-Control header to indicate how many seconds this response can be cached. This can be particularly useful if you are using varnish on your server.


Set the Expires header to the year 2037. You can use this with content which should never expire, such as when the request path has a hash value associated with it.


Sets the Expires header to the past.


Sets the Expires header to the specified date/time.

I/O and debugging

The HandlerT and WidgetT monad transformers are both instances of a number of typeclasses. For this section, the important typeclasses are MonadIO and MonadLogger. The former allows you to perform arbitrary IO actions inside your handler, such as reading from a file. In order to achieve this, you just need to prepend liftIO to the call.

MonadLogger provides a built-in logging system. There are many ways you can customize this system, including what messages get logged and where logs are sent. By default, logs are sent to standard output, in development all messages are logged, and in production, warnings and errors are logged.

Often times when logging, we want to know where in the source code the logging occured. For this, MonadLogger provides a number of convenience Template Haskell functions which will automatically insert source code location into the log messages. These functions are $logDebug, $logInfo, $logWarn, and $logError. Let’s look at a short example of some of these functions.

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes       #-}
{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE TypeFamilies      #-}
import           Control.Exception (IOException, try)
import           Control.Monad     (when)
import           Yesod

data App = App

mkYesod "App" [parseRoutes|
/ HomeR GET

instance Yesod App where
    -- This function controls which messages are logged
    shouldLogIO App src level =
        return True -- good for development
        -- level == LevelWarn || level == LevelError -- good for production

getHomeR :: Handler Html
getHomeR = do
    $logDebug "Trying to read data file"
    edata <- liftIO $ try $ readFile "datafile.txt"
    case edata :: Either IOException String of
        Left e -> do
            $logError "Could not read datafile.txt"
            defaultLayout [whamlet|An error occurred|]
        Right str -> do
            $logInfo "Reading of data file succeeded"
            let ls = lines str
            when (length ls < 5) $ $logWarn "Less than 5 lines of data"
                        $forall l <- ls

main :: IO ()
main = warp 3000 App

Query string and hash fragments

We’ve looked at a number of functions which work on URL-like things, such as redirect. These functions all work with type-safe URLs, but what else do they work with? There’s a typeclass called RedirectUrl which contains the logic for converting some type into a textual URL. This includes type-safe URLs, textual URLs, and two special instances:

  1. A tuple of a URL and a list of key/value pairs of query string parameters.

  2. The Fragment datatype, used for adding a hash fragment to the end of a URL.

Both of these instances allow you to "add on" extra information to a type-safe URL. Let’s see some examples of how these can be used:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes       #-}
{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE TypeFamilies      #-}
import           Data.Text        (Text)
import           Yesod

data App = App

mkYesod "App" [parseRoutes|
/      HomeR  GET
/link1 Link1R GET
/link2 Link2R GET
/link3 Link3R GET
/link4 Link4R GET

instance Yesod App where

getHomeR :: Handler Html
getHomeR = defaultLayout $ do
    setTitle "Redirects"
            <a href=@{Link1R}>Click to start the redirect chain!

getLink1R, getLink2R, getLink3R :: Handler ()
getLink1R = redirect Link2R -- /link2
getLink2R = redirect (Link3R, [("foo", "bar")]) -- /link3?foo=bar
getLink3R = redirect $ Link4R :#: ("baz" :: Text) -- /link4#baz

getLink4R :: Handler Html
getLink4R = defaultLayout
        <p>You made it!

main :: IO ()
main = warp 3000 App

Of course, inside a Hamlet template this is usually not necessary, as you can simply include the hash after the URL directly, e.g.:

<a href=@{Link1R}#somehash>Link to hash


Routing and dispatch is arguably the core of Yesod: it is from here that our type-safe URLs are defined, and the majority of our code is written within the Handler monad. This chapter covered some of the most important and central concepts of Yesod, so it is important that you properly digest it.

This chapter also hinted at a number of more complex Yesod topics that we will be covering later. But you should be able to write some very sophisticated web applications with just the knowledge you have learned up until here.