I Want This (Order)

Snap for Beginners

Sample Chapter: Digestive Functors (Form Processing)


Snap allows writing routes in fairly familiar way. It then takes these routes in the addRoutes function and turns them into a trie that gives us O(log n) dispatching time.

Route Definitions

The route definitions from code/routing-app/src/Site.hs:

routes :: [(ByteString, Handler App App ())]
routes = [ ("/logins",    with auth handleLoginSubmit)
         , ("/logout",   with auth handleLogout)
         , ("/new_user", with auth handleNewUser)
         , ("",          serveDirectory "static")

which we then add in our app initialization:

addRoutes routes

We can see that we define our routes with a list of tuples. Each tuple consists of a URL fragment and a function.

Similar routes are combined using Control.Applicative's Alternative class (<|>). To get some basic intuition for how <|> works, we can run some experiments in ghci.

Enter ghci:


Import Control.Applicative

:m Control.Applicative

Now we can use the <|> operator to test. In this example > is used to represent the prompt, everything after > is typed into ghci and content without a > at the beginning is the return value of the previous line.

>Nothing <|> Just 4
Just 4
>Nothing <|> Just 4 <|> Just 5
Just 4
> Nothing <|> Nothing <|> Just 5
Just 5
>Nothing <|> Nothing <|> Nothing

We use <|> later in this chapter to match routes based on method.


Parameters can be in three places: rqQueryParams for the query string, rqPostParams for POST bodies and rqParams for a union of the two previous maps.

URL Parameters

We can also use the :paramname form in the route to get parameters from the URI. We'll use a sample handler to echo back the parameter in the url:

echoHandler :: Handler App App ()
echoHandler = do
  param <- getParam "echoparam"
  maybe (writeBS "must specify echo/param in URL")
         writeBS param

getParam will get the parameter from either a GET or POST request and then we respond with either "must specify echo/param in URL" if there is no param or the value of the param. Here is the route we use:

routes = [("/echo/:echoparam", echoHandler)]

It will be used for /echo/something/ and for /echo/something/many/things/ but not for /echo/. Both times it will respond with "something".


We can solve this issue with ifTop. We can create a second route that will only respond to the base route we define, in this case /echotwo/parameter. /echotwo/ and /echotwo/something/anythinghere/ will fail.

("/echotwo/:echoparam", ifTop echoHandler)

method VERB

For additional restriction we can use method. method allows us to restrict route handlers to specific verbs, such as GET or POST.

We can define two handlers for GET and POST that simply respond with "getHandler" and "postHandler" respectively.

getHandler :: Handler App App ()
getHandler = writeBS "getHandler"

postHandler :: Handler App App ()
postHandler = writeBS "postHandler"

We can then set up method GET getHandler, which will only run GET requests to the getHandler we can then chain it with method POST postHandler using <|>. Note that this will behave very similar to the example at the beginning of the chapter. Behind the scenes method uses unless from Control.Monad to determine whether or not to "pass" to the next handler.

("/getorpost", method GET getHandler <|> method POST postHandler)

We could then run curl to test the routes:

curl localhost:8000/getorpost

should return "getHandler" while:

curl -XPOST localhost:8000/getorpost -d "stuff"

will return "postHandler".

Sending Data Back

There are many ways to send data in the response. A few of them are here. If we use the OverloadedStrings language pragma we can write string literals as below. If we don't we would have to write the respective pack functions for each data type.

It is important to note that writeBS doesn't actually write to the socket, but rather adds to the closure in the Response that will be called. This allows us to use multiple calls to writeBS in the same handler. The 1.0 release of Snap will be based on streams (using io-streams). A future version of this book will cover that.


Writes a ByteString back to the client.

writeBS "data here"


Writes Text back to the client.

writeText "data here"


writeJSON is from the Snap.Extras.JSON package and can be used in conjunction with Data.Aeson to more easily write JSON responses. It will set the MIME to 'application/json' and write the given object into the response body.

If we have a custom datatype and a ToJSON instance from Data.Aeson we can use writeJSONto send it as a JSON representation. Fromcode/routing-app/src/Site.hs`:

data Person  = Person {
  name :: String
  } deriving (Show)
instance ToJSON Person where
  toJSON (Person s) = object ["name" .= s]

and in our route/handler we create a new Person and pass it to writeJSON:

("/json", writeJSON $ Person "me")

When we hit http://localhost:8000/json we should get: