Authentication infrastructure for the Snap Framework (NOTE: This package is deprecated. As of 0.6 this functionality is included with snap)
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Latest commit 45294e8 Jun 23, 2011 @ozataman ozataman Bump Snap version to 0.5

Snap-auth provides authentication and session management functionality for Snap. Eventually this will probably be moved into the snap package. But we're starting it off in a separate package until we get a better sense of how snap code will be organized.

The Concept

User/session management has two basic levels (potentially more if you add permissions/roles/etc.):

  • Making sure an established session between any user - authenticated or otherwise - and the server stays secure.
  • Authenticating users, which means having proof that a user is who she says she is before we grant her some important priveleges in our application.

This package both of these challenges. It will likely be integrated into Snap as the stock solution, possibly in the 0.5 release.

Session Management

First, let's demonstrate the session management piece.


For those familiar with Rails, the functionality is similar to

session[:user_id] = 1234
session[:last_query] = "johnpollak"

The difference, however, is that we can't just store arbitrary data -types and instead use only ByteStrings.

We define a type Session as

type Session = Map ByteString ByteString

which gives us all the convenience and power of Haskell's standard Map library.

It is yet to be seen if this is effective and/or efficient in the long run but has worked well so far.

Setting Up Your Application With Sessions

Let's setup the session functionality using the CookieSession backend.

-- Define a field to hold the session state in your application state
data ApplicationState = ApplicationState
  { appSessionSt :: CookieSessionState }

-- Instantiate your app as a MonadSession
instance HasCookieSessionState ApplicationState where
  getCookieSessionState = appSessionSt

-- Add some simple initializer code
appInit :: Initializer ApplicationState
appInit = do
  cs <- cookieSessionStateInitializer $ defCookieSessionState
          { csKeyPath = "config/site-key.txt" 
          , csCookieName = "myapp-session" }
  return $ ApplicationState cs

And you are done. While you have to do this manually for now, we will in the future have the snap executable auto-generate some of this boiler plate for you.

Usage Example

Let's assume we have an odd desire to persist our user's age in our session store:

import qualified Data.Map as M
import           Snap.Extension.CookieSession


myHandler = do
  setInSession "user_age" "32" -- that's all we have to do!
  render "pages/myPage"

The "user_age" field will now be available in this user's session until we delete it or expire the session.

We can now retrieve it at any point with:

myHandler2 = do
  uage <- getFromSession "user_age"
  doSomethingWithUid uage
  render "pages/myPage2"



There is currently a single back-end: Snap.Extension.Session.CookieSession. It uses Data.Serialize to serialize the Session data type and Michael Snoyman's Web.ClientSession to encrypt the cookie. The cookie is encrypted, which means it is fully secure and can't be read by the client/end-user.

Since this method has no need for a DB back-end, it works out of the box and is pretty much the simplest session persistence back-end to use. For those familiar, this method is the default behavior in Ruby on Rails as well.

Please see the Haddock documentation for more information.

Other Backends

The idea would be to add various other back-ends as desired. Redis, MongoDB, SQL-based databases, etc. should all be straightforward enough to implement. We would just need a scheme to presist the session type in the respective database.


The second layer of thic package provides for user athentication. It defines an AuthUser datatype that holds all of the core authentication fields for a "user". Let's look at it so we can get a sense for what is possible:

data AuthUser = AuthUser 
  { userId :: Maybe UserId
  , userEmail :: Maybe ByteString
  , userPassword :: Maybe Password
  , userSalt :: Maybe ByteString
  , userActivatedAt :: Maybe UTCTime
  , userSuspendedAt :: Maybe UTCTime
  , userLoginCount :: Int
  , userFailedLoginCount :: Int
  , userCurrentLoginAt :: Maybe UTCTime
  , userLastLoginAt :: Maybe UTCTime
  , userCurrentLoginIp :: Maybe ByteString
  , userLastLoginIp :: Maybe ByteString
  , userCreatedAt :: Maybe UTCTime
  , userUpdatedAt :: Maybe UTCTime
  } deriving (Read,Show,Ord,Eq)

The authentication piece has two key typeclasses that we need to be aware of.

MonadAuth Typeclass

To enable authentication, we need to make our application monad an instance of MonadAuth. While doing so, we get to choose/customize various authentication parameters. The simplest way to instantiate our application is simply:

instance MonadAuth Application

and done. That's right, we have all the sensible defaults set up so you could potentially just do that. More typically, here is what you would specify:

instance MonadAuth Application where
  authAuthenticationKeys = return ["login", "domain"]
  authUserTable = return "myusers"

and so on. Take a look at haddocks to see what can be specified.

NOTE: We are still working on implementing some of these options, but it should be complete soon enough.

MonadAuthUser Typeclass

Now onto the database integration. This typeclass is all about persisting users in some form of storage. Whatever snap database extension is being used would be expected to instantiate this typeclass and have nice integration with MonadAuth.

As an example, Snap.Extension.DB.MongoDB has ongoing support for MonadAuth and instantiates MonadAuthUser for free. See the repo at:

A couple of key ideas to understand this typeclass are as follows:

  1. User can be looked up in 2 ways:
    • With an internal/db-provided unique bytestring identifier. This is the "id" field in most db systems.
    • A Map of key, value pairs that can be used to look up a user in the db. This is the external interface and is typically submitted through a web form. This is how the user of you application will identify herself during login.
  2. The user table in the DB can contain more fields than necessary for authentication. This is both natural and typical. So the saveAuthUser function takes a (AuthUser, t) input. AuthUser contains the core authentication fields and t is passed directly to the DB back-end to be included in the save. As an example, in MongoDB implementation t is the Document datatype and is merged with the AuthUser fields prior to database save.

Again, this typeclass is instantiated by the DB extension you are using, so normally you should not need to implement it.

Usage Example

Here is a simple example. We'll provide more thorough documentation as things crystallize.

data User = User
  { authUser :: AuthUser
  , myField1 :: ByteStrings
  , myField2 :: ByteStrings

-- Construct your 'User' from the given parameters
-- Make sure you do validation as well - at least for now.
makeUser ps = return $ User { .... }

additionalUserFields :: User -> Document
additionalUserFields u = [ "myField1" =: myField1 u
                         , "myField2" =: myField2 u ]

site = routes $
  [ ("/signup", method GET $ newSignupH)
  , ("/signup", method POST $ signupH)

  , ("/login", method GET $ newSessionH)
  , ("/login", method POST $ loginHandler "password" newSessionH redirHome)

redirHome = redirect "/"

-- Make sure you have a 'password' field in there
newSessionH = render "login"

-- Assuming you have a signup.tpl template
newSignupH = render "signup"

-- Save user and redirect as appropriate
signupH :: Application ()
signupH = do
  ps <- getParams
  let u = makeUser ps
  au <- saveAuthUser (u, additionalUserFields u)
  case au of
    Nothing -> newSignupH
    Just au' -> do setSessionUserId $ userId au'
                   redirect "/"




  • Splices/handlers for easy CSRF protection token integration:
    • csrf_meta_tag for unobtrusive JS based binding to forms (like in Rails 3)
    • csrf_token_tag for a hidden field inside forms (in progress)
    • verify_authenticity handler to be chained before your destructive handlers

Planned Back-ends

  • MongoDB backend
  • HDBC-based SQL back-ends once extension-hdbc is in place

Open Questions/Considerations

  • Possibility of using JSON-like datatype for session store.


  • Challenge/response authentication ( This is needed to provide secure authentication without SSL. The goal is to take as much of the burden as possible off the end user, which probably means including some Javascript code for use on the client side. If the client is not javascript-enabled, then the user should have the option to failover seamlessly to less secure authentication (that transmits cleartext passwords across the network) or alert the user and disallow logins..

  • Support for "remember me" and "password reset" tokens.

  • Perhaps make Password / Salt opaque field with a Show/Read instance for DB serialization - users should really never need that stuff