servant-auth.hs

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}

import Data.Aeson                       (ToJSON)
import Data.ByteString                  (ByteString)
import Data.Map                         (Map, fromList)
import Data.Monoid                      ((<>))
import qualified Data.Map            as Map
import Data.Proxy                       (Proxy (Proxy))
import Data.Text                        (Text)
import GHC.Generics                     (Generic)
import Network.Wai                      (Request, requestHeaders)
import Network.Wai.Handler.Warp         (run)
import Servant.API                      ((:<|>) ((:<|>)), (:>), BasicAuth,
                                          Get, JSON)
import Servant.API.BasicAuth            (BasicAuthData (BasicAuthData))
import Servant.API.Experimental.Auth    (AuthProtect)
import Servant                          (throwError)
import Servant.Server                   (BasicAuthCheck (BasicAuthCheck),
                                         BasicAuthResult( Authorized
                                                        , Unauthorized
                                                        ),
                                         Context ((:.), EmptyContext),
                                         err401, err403, errBody, Server,
                                         serveWithContext, Handler)
import Servant.Server.Experimental.Auth (AuthHandler, AuthServerData,
                                         mkAuthHandler)
import Servant.Server.Experimental.Auth()

----------------------------------------------------------------------------
-- API
----------------------------------------------------------------------------

-- | private data that needs protection
newtype PrivateData = PrivateData { ssshhh :: Text }
  deriving (Eq, Show, Generic)

instance ToJSON PrivateData

-- | public data that anyone can use.
newtype PublicData = PublicData { somedata :: Text }
  deriving (Eq, Show, Generic)

instance ToJSON PublicData

-- | A user we'll grab from the database when we authenticate someone
newtype User = User { userName :: Text }
  deriving (Eq, Show)

-- | a type to wrap our public api
type PublicAPI = Get '[JSON] [PublicData]

-- | a type to wrap our private api
type PrivateAPI = Get '[JSON] PrivateData

-- | our API
type BasicAPI = "public"  :> PublicAPI
           :<|> "private" :> BasicAuth "foo-realm" User :> PrivateAPI

-- | a value holding a proxy of our API type
basicAuthApi :: Proxy BasicAPI
basicAuthApi = Proxy

----------------------------------------------------------------------------
-- Authentication
----------------------------------------------------------------------------

-- | A user type that we "fetch from the database" after
-- performing authentication
newtype Account = Account { unAccount :: Text }

-- | A (pure) database mapping keys to users.
database :: Map ByteString Account
database = fromList [ ("key1", Account "Anne Briggs")
                    , ("key2", Account "Bruce Cockburn")
                    , ("key3", Account "Ghédalia Tazartès")
                    ]

-- | A method that, when given a password, will return a Account.
-- This is our bespoke (and bad) authentication logic.
lookupAccount :: ByteString -> Handler Account
lookupAccount key = case Map.lookup key database of
  Nothing -> throwError (err403 { errBody = "Invalid Cookie" })
  Just usr -> return usr

-- | The auth handler wraps a function from Request -> Handler Account
-- we look for a Cookie and pass the value of the cookie to `lookupAccount`.
authHandler :: AuthHandler Request Account
authHandler =
  let handler req = case lookup "servant-auth-cookie" (requestHeaders req) of
        Nothing -> throwError (err401 { errBody = "Missing auth header" })
        Just authCookieKey -> lookupAccount authCookieKey
  in mkAuthHandler handler

----------------------------------------------------------------------------
-- API with authentication
----------------------------------------------------------------------------

-- | Our API, with auth-protection
type AuthGenAPI = "private" :> AuthProtect "cookie-auth" :> PrivateAPI
             :<|> "public"  :> PublicAPI

-- | A value holding our type-level API
genAuthAPI :: Proxy AuthGenAPI
genAuthAPI = Proxy

-- | We need to specify the data returned after authentication
type instance AuthServerData (AuthProtect "cookie-auth") = Account

----------------------------------------------------------------------------
-- Run everything
----------------------------------------------------------------------------

-- | The context that will be made available to request handlers. We supply the
-- "cookie-auth"-tagged request handler defined above, so that the 'HasServer' instance
-- of 'AuthProtect' can extract the handler and run it on the request.
genAuthServerContext :: Context (AuthHandler Request Account ': '[])
genAuthServerContext = authHandler :. EmptyContext

-- | Our API, where we provide all the author-supplied handlers for each end
-- point. Note that 'privateDataFunc' is a function that takes 'Account' as an
-- argument. We dont' worry about the authentication instrumentation here,
-- that is taken care of by supplying context
genAuthServer :: Server AuthGenAPI
genAuthServer =
  let privateDataFunc (Account name) =
          return (PrivateData ("this is a secret: " <> name))
      publicData = return [PublicData "this is a public piece of data"]
  in  privateDataFunc :<|> publicData

-- | run our server
genAuthMain :: IO ()
genAuthMain = run 8080 (serveWithContext genAuthAPI genAuthServerContext genAuthServer)

{- Sample Session:

$ curl -XGET localhost:8080/private
Missing auth header

$ curl -XGET localhost:8080/private -H "servant-auth-cookie: key3"
[{"ssshhh":"this is a secret: Ghédalia Tazartès"}]

$ curl -XGET localhost:8080/private -H "servant-auth-cookie: bad-key"
Invalid Cookie

$ curl -XGET localhost:8080/public
[{"somedata":"this is a public piece of data"}]
-}