A family of combinators for defining webservices APIs and serving them

Latest on Hackage:0.8.2

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow to host generated Haddocks.

BSD3 licensed by Alp Mestanogullari, Sönke Hahn, Julian K. Arni, Greg Hale
Maintained by


Build Status

A library for running servant APIs under any MonadSnap monad. For example, we can build a Servant server from Snap handlers:

> :set -XTypeOperators -XDataKinds -XOverloadedStrings
> import Data.Proxy (Proxy(..))
> import Data.Text (Text, append)
> import Servant
> import Servant.Server (serveSnap)
> import Snap.Http.Server (quickHttpServe)
> type Api = "number" :> Get '[JSON] Int  :<|>  "hello" :> Capture "name" String :> Get '[JSON] String)
> let api = Proxy :: Proxy Api
> quickHttpServe $ serveSnap api (return 5 :<|> \name -> return (append "Hello, " name))

curl localhost:8000/hello/world
"Hello, world"

Or we can integrate a Servant API into a more complex application with Snaplets and your own Handler b v monad.

There is a longer example demonstrating this below, and one you can compile at example/greet.hs.

For more information about getting started servant itself, please see the servant documentation. To learn about Snap, see the snap-core and snaplet haddocks.

Comparison with servant-server

servant-snap is very similar to servant-server, providing a number of HasServer instances for various servant API combinators and the function serveSnap for turning your API spec into a runnable server.

servant-snap hosts MonadSnap m handlers (for example Snap, Handler b v). servant-server hosts EitherT ServantErr IO handlers and other monads that can be naturally transformed via [enter]( This makes servant-snap both a bit more flexible and a bit less type-safe, because servant-snap handlers can access data from the HTTP request. servant-server on the other hand hides the raw request from the handlers so that all routing decisions are determined strictly by the API type.

Snap’s Handler b v handlers use lenses and snaplets to control which effects and state the handler can access. In servant-server, you use [vault]( to hold these needed bits of state (for example, database connections).

servant-server provides combinators for controlling access to sub-api’s. servant-snap still does authentication through the [auth snaplet](, without signalling this in the API type. We are still thinking about how to support type-level signalling of auth requirements for servant-snap. It would be possible to provide snap-specific auth combinators, but using them would force a snap dependency on the API itself, which we want to avoid (it is common to compile a servant API with ghcjs, so we want the API to be packaged without any system dependencies).


Can servant-snap serve Heist templates?

You can call renderTemplate in a handler for a Raw endpoint. We don’t offer a Heist combinator, but in the future this may be nice to have in another package.

What does Snap offer beyond a regular servant-server application?

Snap has a very simple core, a nice monad for request/response handling, and a large number of stable units of added functionality (“Snaplets”) for handling auth, persistence, templating, etc. One of the biggest pain-points of Snap use is manually digging data out of requests, validating it, and building responses. Servant fills this gap for snap perfectly. Read more about the Snap Framework!

If this package is called servant-snap, shouldn’t servant-server be named servant-wai? The current naming is confusing.

At some point, we hope to split servant-server into a generic base part and servant-wai, servant-snap, backends. This would make writing other backends easier and drastically reduce code duplication.

More detailed Snap example

Note: It’s recommended to use snap init to build a site template for new projects, rather than packing everything into a single file like this.

-- Top-level Application State
data App = App
  { _heist :: Snaplet (Heist App)
  , _sess  :: Snaplet SessionManager
  , _db    :: Snaplet Postgres
  , _auth  :: Snaplet (AuthManager App)

makeLenses'' App

instance HasHeist App where
  heistLens = subSnaplet heist

-- Our app's base monad
type AppHandler = Handler App App

-- Our Servant API
type Api = "users" :> Get '[JSON] [User]
           "user" :> Capture "name" Text :> Get '[JSON] User
           "reset_db" :> Post '[JSON] NoContent

-- Servant API implementation using DB and Auth Snaplets
apiServer :: ServerT Api AppHandler
apiServer = getAllUsers :<|> getUserByName :<|> resetDB
    getAllUsers =
      with db $ query_ "SELECT * FROM users"

    getUserByName name =
      with db $ query  "SELECT * FROM users WHERE name=(?)" [(Only name)]

    resetDB = do
      isAdmin <- with auth $ do
        u <- getCurrentUser
        return ((userLogin <$> u) == Just "admin")
      if isAdmin
      then with db $ query_ "DELETE FROM users"
      else throwError err403

-- Snap initializer action
-- (Notice that we mix servant and non-servant routes)
app :: SnapletInit App App
app =  makeSnaplet "app" "standard example app" Nothing $ do
  h <- nestSnaplet "" heist $  heistInit "templates"
  s <- nestSnaplet "sess" sess $
    initCookieSessionManager "site_key.txt" "sess" Nothing (Just 3600)
  d <- nestSnaplet "db" db pgsInit
  a <- nestSnaplet "auth" auth $ initPostgresAuth sess d
  addRoutes [ ("login",     with auth handleLoginSubmit)    -- Regular snap handler
            , ("logout",    with auth handleLogout)         -- for non-API routes
            , ("new_user",  with auth handleNewUser)
            , ("api",       serveSnap api apiServer         -- servant-snap handler
            , ("",          serveDirectory "static")
  addAuthSplices h auth
  return $ App h s d a

main :: IO ()
main = httpServe defaultConfig app



Add HasServer instances for StreamGenerator

Add headers from MonadSnap state response to the servant-snap computed response Add a commented-out snap-cors test to the test suite. It doesn’t pass, although manual testing of snap-cors works.


Copy BasicAuth and Context from servant-server to support basic auth checking


Call ‘Snap.Core.pass’ when routing an empty URI path. This allows an entire served API to fall through, which is more in line with the rest of snap routing, and allows multiple servant API’s to be served under the same path context from ‘Snap.Core.route’.

Fix throwError bug ignoring ServantError headers

Fix throwError bug ignoring ServantError body

Bump servant upper bound, allow 0.9


Initial release

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