yesod/Yesod/Handler.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE CPP #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
---------------------------------------------------------
--
-- Module        : Yesod.Handler
-- Copyright     : Michael Snoyman
-- License       : BSD3
--
-- Maintainer    : Michael Snoyman <michael@snoyman.com>
-- Stability     : unstable
-- Portability   : portable
--
-- Define Handler stuff.
--
---------------------------------------------------------
module Yesod.Handler
    ( -- * Type families
      Route
    , YesodSubRoute (..)
      -- * Handler monad
    , GHandler
    , GGHandler
      -- ** Read information from handler
    , getYesod
    , getYesodSub
    , getUrlRender
    , getUrlRenderParams
    , getCurrentRoute
    , getRouteToMaster
    , getRequest
    , waiRequest
    , runRequestBody
      -- * Special responses
      -- ** Redirecting
    , RedirectType (..)
    , redirect
    , redirectParams
    , redirectString
    , redirectText
    , redirectToPost
      -- ** Errors
    , notFound
    , badMethod
    , permissionDenied
    , invalidArgs
      -- ** Short-circuit responses.
    , sendFile
    , sendFilePart
    , sendResponse
    , sendResponseStatus
    , sendResponseCreated
    , sendWaiResponse
      -- * Setting headers
    , setCookie
    , deleteCookie
    , setHeader
    , setLanguage
      -- ** Content caching and expiration
    , cacheSeconds
    , neverExpires
    , alreadyExpired
    , expiresAt
      -- * Session
    , SessionMap
    , lookupSession
    , getSession
    , setSession
    , deleteSession
      -- ** Ultimate destination
    , setUltDest
    , setUltDestString
    , setUltDest'
    , redirectUltDest
      -- ** Messages
    , setMessage
    , getMessage
      -- * Helpers for specific content
      -- ** Hamlet
    , hamletToContent
    , hamletToRepHtml
      -- ** Misc
    , newIdent
    , liftIOHandler
      -- * Internal Yesod
    , runHandler
    , YesodApp (..)
    , runSubsiteGetter
    , toMasterHandler
    , toMasterHandlerDyn
    , toMasterHandlerMaybe
    , localNoCurrent
    , HandlerData
    , ErrorResponse (..)
    , YesodAppResult (..)
    , handlerToYAR
    , yarToResponse
    , headerToPair
    ) where

import Prelude hiding (catch)
import Yesod.Internal.Request
import Yesod.Internal
import Data.Time (UTCTime)

import Control.Exception hiding (Handler, catch, finally)
import qualified Control.Exception as E
import Control.Applicative

import Control.Monad (liftM, join)

import Control.Monad.IO.Class
import Control.Monad.Trans.Class
import Control.Monad.Trans.Writer
import Control.Monad.Trans.Reader
import Control.Monad.Trans.State
import Control.Monad.Trans.Error (throwError, ErrorT (..), Error (..))

import System.IO
import qualified Network.Wai as W
import qualified Network.HTTP.Types as H
import Control.Failure (Failure (failure))

import Text.Hamlet
import Text.Blaze (preEscapedText)
import qualified Text.Blaze.Renderer.Text
import qualified Data.Text as T
import Data.Text.Encoding (encodeUtf8, decodeUtf8With)
import Data.Text.Encoding.Error (lenientDecode)
import qualified Data.Text.Lazy as TL

import Control.Monad.IO.Control (MonadControlIO)
import Control.Monad.Trans.Control (MonadTransControl, liftControl)
import qualified Data.Map as Map
import qualified Data.ByteString as S
import Data.ByteString (ByteString)
import Data.Enumerator (Iteratee (..))
import Network.Wai.Parse (parseHttpAccept)

import Yesod.Content
import Data.Maybe (fromMaybe)
import Web.Cookie (SetCookie (..), renderSetCookie)
import Data.Enumerator (run_, ($$))
import Control.Arrow (second, (***))
import qualified Network.Wai.Parse as NWP
import Data.Monoid (mappend, mempty)
import qualified Data.ByteString.Char8 as S8
import Data.CaseInsensitive (CI)
import Blaze.ByteString.Builder (toByteString)
import Data.Text (Text)

-- | The type-safe URLs associated with a site argument.
type family Route a

class YesodSubRoute s y where
    fromSubRoute :: s -> y -> Route s -> Route y

data HandlerData sub master = HandlerData
    { handlerRequest :: Request
    , handlerSub :: sub
    , handlerMaster :: master
    , handlerRoute :: Maybe (Route sub)
    , handlerRender :: (Route master -> [(Text, Text)] -> Text)
    , handlerToMaster :: Route sub -> Route master
    }

handlerSubData :: (Route sub -> Route master)
               -> (master -> sub)
               -> Route sub
               -> HandlerData oldSub master
               -> HandlerData sub master
handlerSubData tm ts = handlerSubDataMaybe tm ts . Just

handlerSubDataMaybe :: (Route sub -> Route master)
                    -> (master -> sub)
                    -> Maybe (Route sub)
                    -> HandlerData oldSub master
                    -> HandlerData sub master
handlerSubDataMaybe tm ts route hd = hd
    { handlerSub = ts $ handlerMaster hd
    , handlerToMaster = tm
    , handlerRoute = route
    }

-- | Used internally for promoting subsite handler functions to master site
-- handler functions. Should not be needed by users.
toMasterHandler :: (Route sub -> Route master)
                -> (master -> sub)
                -> Route sub
                -> GGHandler sub master mo a
                -> GGHandler sub' master mo a
toMasterHandler tm ts route (GHandler h) =
    GHandler $ withReaderT (handlerSubData tm ts route) h

toMasterHandlerDyn :: Monad mo
                   => (Route sub -> Route master)
                   -> GGHandler sub' master mo sub
                   -> Route sub
                   -> GGHandler sub master mo a
                   -> GGHandler sub' master mo a
toMasterHandlerDyn tm getSub route (GHandler h) = do
    sub <- getSub
    GHandler $ withReaderT (handlerSubData tm (const sub) route) h

class SubsiteGetter g m s | g -> s where
  runSubsiteGetter :: g -> m s

instance (master ~ master'
         ) => SubsiteGetter (master -> sub) (GHandler anySub master') sub where
  runSubsiteGetter getter = do
    y <- getYesod
    return $ getter y

instance (anySub ~ anySub'
         ,master ~ master'
         ) => SubsiteGetter (GHandler anySub master sub) (GHandler anySub' master') sub where
  runSubsiteGetter = id

toMasterHandlerMaybe :: (Route sub -> Route master)
                     -> (master -> sub)
                     -> Maybe (Route sub)
                     -> GGHandler sub master mo a
                     -> GGHandler sub' master mo a
toMasterHandlerMaybe tm ts route (GHandler h) =
    GHandler $ withReaderT (handlerSubDataMaybe tm ts route) h

-- | A generic handler monad, which can have a different subsite and master
-- site. This monad is a combination of 'ReaderT' for basic arguments, a
-- 'WriterT' for headers and session, and an 'MEitherT' monad for handling
-- special responses. It is declared as a newtype to make compiler errors more
-- readable.
newtype GGHandler sub master m a =
    GHandler
        { unGHandler :: GHInner sub master m a
        }
    deriving (Functor, Applicative, Monad, MonadIO, MonadControlIO)

instance MonadTrans (GGHandler s m) where
    lift = GHandler . lift . lift . lift . lift

type GHandler sub master = GGHandler sub master (Iteratee ByteString IO)

data GHState = GHState
    { ghsSession :: SessionMap
    , ghsRBC :: Maybe RequestBodyContents
    , ghsIdent :: Int
    }

type GHInner s m monad = -- FIXME collapse the stack
    ReaderT (HandlerData s m) (
    ErrorT HandlerContents (
    WriterT (Endo [Header]) (
    StateT GHState (
    monad
    ))))

type SessionMap = Map.Map Text Text

type Endo a = a -> a

-- | An extension of the basic WAI 'W.Application' datatype to provide extra
-- features needed by Yesod. Users should never need to use this directly, as
-- the 'GHandler' monad and template haskell code should hide it away.
newtype YesodApp = YesodApp
    { unYesodApp
    :: (ErrorResponse -> YesodApp)
    -> Request
    -> [ContentType]
    -> SessionMap
    -> Iteratee ByteString IO YesodAppResult
    }

data YesodAppResult
    = YARWai W.Response
    | YARPlain H.Status [Header] ContentType Content SessionMap

data HandlerContents =
      HCContent H.Status ChooseRep
    | HCError ErrorResponse
    | HCSendFile ContentType FilePath (Maybe W.FilePart) -- FIXME replace FilePath with opaque type from system-filepath?
    | HCRedirect RedirectType Text
    | HCCreated Text
    | HCWai W.Response

instance Error HandlerContents where
    strMsg = HCError . InternalError . T.pack

getRequest :: Monad mo => GGHandler s m mo Request
getRequest = handlerRequest `liftM` GHandler ask

instance Monad monad => Failure ErrorResponse (GGHandler sub master monad) where
    failure = GHandler . lift . throwError . HCError

runRequestBody :: GHandler s m RequestBodyContents
runRequestBody = do
    x <- GHandler $ lift $ lift $ lift get
    case ghsRBC x of
        Just rbc -> return rbc
        Nothing -> do
            rr <- waiRequest
            rbc <- lift $ rbHelper rr
            GHandler $ lift $ lift $ lift $ put x { ghsRBC = Just rbc }
            return rbc

rbHelper :: W.Request -> Iteratee ByteString IO RequestBodyContents
rbHelper req =
    (map fix1 *** map fix2) <$> iter
  where
    iter = NWP.parseRequestBody NWP.lbsSink req
    fix1 = go *** go
    fix2 (x, NWP.FileInfo a b c) =
        (go x, FileInfo (go a) (go b) c)
    go = decodeUtf8With lenientDecode

-- | Get the sub application argument.
getYesodSub :: Monad m => GGHandler sub master m sub
getYesodSub = handlerSub `liftM` GHandler ask

-- | Get the master site appliation argument.
getYesod :: Monad m => GGHandler sub master m master
getYesod = handlerMaster `liftM` GHandler ask

-- | Get the URL rendering function.
getUrlRender :: Monad m => GGHandler sub master m (Route master -> Text)
getUrlRender = do
    x <- handlerRender `liftM` GHandler ask
    return $ flip x []

-- | The URL rendering function with query-string parameters.
getUrlRenderParams
    :: Monad m
    => GGHandler sub master m (Route master -> [(Text, Text)] -> Text)
getUrlRenderParams = handlerRender `liftM` GHandler ask

-- | Get the route requested by the user. If this is a 404 response- where the
-- user requested an invalid route- this function will return 'Nothing'.
getCurrentRoute :: Monad m => GGHandler sub master m (Maybe (Route sub))
getCurrentRoute = handlerRoute `liftM` GHandler ask

-- | Get the function to promote a route for a subsite to a route for the
-- master site.
getRouteToMaster :: Monad m => GGHandler sub master m (Route sub -> Route master)
getRouteToMaster = handlerToMaster `liftM` GHandler ask

-- | Function used internally by Yesod in the process of converting a
-- 'GHandler' into an 'W.Application'. Should not be needed by users.
runHandler :: HasReps c
           => GHandler sub master c
           -> (Route master -> [(Text, Text)] -> Text)
           -> Maybe (Route sub)
           -> (Route sub -> Route master)
           -> master
           -> sub
           -> YesodApp
runHandler handler mrender sroute tomr ma sa =
  YesodApp $ \eh rr cts initSession -> do
    let toErrorHandler e =
            case fromException e of
                Just x -> x
                Nothing -> InternalError $ T.pack $ show e
    let hd = HandlerData
            { handlerRequest = rr
            , handlerSub = sa
            , handlerMaster = ma
            , handlerRoute = sroute
            , handlerRender = mrender
            , handlerToMaster = tomr
            }
    let initSession' = GHState initSession Nothing 1
    ((contents', headers), finalSession) <- catchIter (
        fmap (second ghsSession)
      $ flip runStateT initSession'
      $ runWriterT
      $ runErrorT
      $ flip runReaderT hd
      $ unGHandler handler
        ) (\e -> return ((Left $ HCError $ toErrorHandler e, id), initSession))
    let contents = either id (HCContent H.status200 . chooseRep) contents'
    let handleError e = do
            yar <- unYesodApp (eh e) safeEh rr cts finalSession
            case yar of
                YARPlain _ hs ct c sess ->
                    let hs' = headers hs
                     in return $ YARPlain (getStatus e) hs' ct c sess
                YARWai _ -> return yar
    let sendFile' ct fp p =
            return $ YARPlain H.status200 (headers []) ct (ContentFile fp p) finalSession
    case contents of
        HCContent status a -> do
            (ct, c) <- liftIO $ chooseRep a cts
            return $ YARPlain status (headers []) ct c finalSession
        HCError e -> handleError e
        HCRedirect rt loc -> do
            let hs = Header "Location" (encodeUtf8 loc) : headers []
            return $ YARPlain
                (getRedirectStatus rt) hs typePlain emptyContent
                finalSession
        HCSendFile ct fp p -> catchIter
            (sendFile' ct fp p)
            (handleError . toErrorHandler)
        HCCreated loc -> do
            let hs = Header "Location" (encodeUtf8 loc) : headers []
            return $ YARPlain
                H.status201
                hs
                typePlain
                emptyContent
                finalSession
        HCWai r -> return $ YARWai r

catchIter :: Exception e
          => Iteratee ByteString IO a
          -> (e -> Iteratee ByteString IO a)
          -> Iteratee ByteString IO a
catchIter (Iteratee mstep) f = Iteratee $ mstep `E.catch` (runIteratee . f)

safeEh :: ErrorResponse -> YesodApp
safeEh er = YesodApp $ \_ _ _ session -> do
    liftIO $ hPutStrLn stderr $ "Error handler errored out: " ++ show er
    return $ YARPlain
        H.status500
        []
        typePlain
        (toContent ("Internal Server Error" :: S.ByteString))
        session

-- | Redirect to the given route.
redirect :: Monad mo => RedirectType -> Route master -> GGHandler sub master mo a
redirect rt url = redirectParams rt url []

-- | Redirects to the given route with the associated query-string parameters.
redirectParams :: Monad mo
               => RedirectType -> Route master -> [(Text, Text)]
               -> GGHandler sub master mo a
redirectParams rt url params = do
    r <- getUrlRenderParams
    redirectString rt $ r url params

-- | Redirect to the given URL.
redirectString, redirectText :: Monad mo => RedirectType -> Text -> GGHandler sub master mo a
redirectText rt = GHandler . lift . throwError . HCRedirect rt
redirectString = redirectText
{-# DEPRECATED redirectString "Use redirectText instead" #-}

ultDestKey :: Text
ultDestKey = "_ULT"

-- | Sets the ultimate destination variable to the given route.
--
-- An ultimate destination is stored in the user session and can be loaded
-- later by 'redirectUltDest'.
setUltDest :: Monad mo => Route master -> GGHandler sub master mo ()
setUltDest dest = do
    render <- getUrlRender
    setUltDestString $ render dest

-- | Same as 'setUltDest', but use the given string.
setUltDestString :: Monad mo => Text -> GGHandler sub master mo ()
setUltDestString = setSession ultDestKey

-- | Same as 'setUltDest', but uses the current page.
--
-- If this is a 404 handler, there is no current page, and then this call does
-- nothing.
setUltDest' :: Monad mo => GGHandler sub master mo ()
setUltDest' = do
    route <- getCurrentRoute
    case route of
        Nothing -> return ()
        Just r -> do
            tm <- getRouteToMaster
            gets' <- reqGetParams `liftM` handlerRequest `liftM` GHandler ask
            render <- getUrlRenderParams
            setUltDestString $ render (tm r) gets'

-- | Redirect to the ultimate destination in the user's session. Clear the
-- value from the session.
--
-- The ultimate destination is set with 'setUltDest'.
redirectUltDest :: Monad mo
                => RedirectType
                -> Route master -- ^ default destination if nothing in session
                -> GGHandler sub master mo ()
redirectUltDest rt def = do
    mdest <- lookupSession ultDestKey
    deleteSession ultDestKey
    maybe (redirect rt def) (redirectText rt) mdest

msgKey :: Text
msgKey = "_MSG"

-- | Sets a message in the user's session.
--
-- See 'getMessage'.
setMessage :: Monad mo => Html -> GGHandler sub master mo ()
setMessage = setSession msgKey . T.concat . TL.toChunks . Text.Blaze.Renderer.Text.renderHtml

-- | Gets the message in the user's session, if available, and then clears the
-- variable.
--
-- See 'setMessage'.
getMessage :: Monad mo => GGHandler sub master mo (Maybe Html)
getMessage = do
    mmsg <- liftM (fmap preEscapedText) $ lookupSession msgKey
    deleteSession msgKey
    return mmsg

-- | Bypass remaining handler code and output the given file.
--
-- For some backends, this is more efficient than reading in the file to
-- memory, since they can optimize file sending via a system call to sendfile.
sendFile :: Monad mo => ContentType -> FilePath -> GGHandler sub master mo a
sendFile ct fp = GHandler . lift . throwError $ HCSendFile ct fp Nothing

-- | Same as 'sendFile', but only sends part of a file.
sendFilePart :: Monad mo
             => ContentType
             -> FilePath
             -> Integer -- ^ offset
             -> Integer -- ^ count
             -> GGHandler sub master mo a
sendFilePart ct fp off count =
    GHandler . lift . throwError $ HCSendFile ct fp $ Just $ W.FilePart off count

-- | Bypass remaining handler code and output the given content with a 200
-- status code.
sendResponse :: (Monad mo, HasReps c) => c -> GGHandler sub master mo a
sendResponse = GHandler . lift . throwError . HCContent H.status200
             . chooseRep

-- | Bypass remaining handler code and output the given content with the given
-- status code.
sendResponseStatus :: (Monad mo, HasReps c) => H.Status -> c -> GGHandler s m mo a
sendResponseStatus s = GHandler . lift . throwError . HCContent s
                     . chooseRep

-- | Send a 201 "Created" response with the given route as the Location
-- response header.
sendResponseCreated :: Monad mo => Route m -> GGHandler s m mo a
sendResponseCreated url = do
    r <- getUrlRender
    GHandler $ lift $ throwError $ HCCreated $ r url

-- | Send a 'W.Response'. Please note: this function is rarely
-- necessary, and will /disregard/ any changes to response headers and session
-- that you have already specified. This function short-circuits. It should be
-- considered only for very specific needs. If you are not sure if you need it,
-- you don't.
sendWaiResponse :: Monad mo => W.Response -> GGHandler s m mo b
sendWaiResponse = GHandler . lift . throwError . HCWai

-- | Return a 404 not found page. Also denotes no handler available.
notFound :: Failure ErrorResponse m => m a
notFound = failure NotFound

-- | Return a 405 method not supported page.
badMethod :: Monad mo => GGHandler s m mo a
badMethod = do
    w <- waiRequest
    failure $ BadMethod $ W.requestMethod w

-- | Return a 403 permission denied page.
permissionDenied :: Failure ErrorResponse m => Text -> m a
permissionDenied = failure . PermissionDenied

-- | Return a 400 invalid arguments page.
invalidArgs :: Failure ErrorResponse m => [Text] -> m a
invalidArgs = failure . InvalidArgs

------- Headers
-- | Set the cookie on the client.
setCookie :: Monad mo
          => Int -- ^ minutes to timeout
          -> H.Ascii -- ^ key
          -> H.Ascii -- ^ value
          -> GGHandler sub master mo ()
setCookie a b = addHeader . AddCookie a b

-- | Unset the cookie on the client.
deleteCookie :: Monad mo => H.Ascii -> GGHandler sub master mo ()
deleteCookie = addHeader . DeleteCookie

-- | Set the language in the user session. Will show up in 'languages' on the
-- next request.
setLanguage :: Monad mo => Text -> GGHandler sub master mo ()
setLanguage = setSession langKey

-- | Set an arbitrary response header.
setHeader :: Monad mo
          => CI H.Ascii -> H.Ascii -> GGHandler sub master mo ()
setHeader a = addHeader . Header a

-- | Set the Cache-Control header to indicate this response should be cached
-- for the given number of seconds.
cacheSeconds :: Monad mo => Int -> GGHandler s m mo ()
cacheSeconds i = setHeader "Cache-Control" $ S8.pack $ concat
    [ "max-age="
    , show i
    , ", public"
    ]

-- | Set the Expires header to some date in 2037. In other words, this content
-- is never (realistically) expired.
neverExpires :: Monad mo => GGHandler s m mo ()
neverExpires = setHeader "Expires" "Thu, 31 Dec 2037 23:55:55 GMT"

-- | Set an Expires header in the past, meaning this content should not be
-- cached.
alreadyExpired :: Monad mo => GGHandler s m mo ()
alreadyExpired = setHeader "Expires" "Thu, 01 Jan 1970 05:05:05 GMT"

-- | Set an Expires header to the given date.
expiresAt :: Monad mo => UTCTime -> GGHandler s m mo ()
expiresAt = setHeader "Expires" . encodeUtf8 . formatRFC1123

-- | Set a variable in the user's session.
--
-- The session is handled by the clientsession package: it sets an encrypted
-- and hashed cookie on the client. This ensures that all data is secure and
-- not tampered with.
setSession :: Monad mo
           => Text -- ^ key
           -> Text -- ^ value
           -> GGHandler sub master mo ()
setSession k = GHandler . lift . lift . lift . modify . modSession . Map.insert k

-- | Unsets a session variable. See 'setSession'.
deleteSession :: Monad mo => Text -> GGHandler sub master mo ()
deleteSession = GHandler . lift . lift . lift . modify . modSession . Map.delete

modSession :: (SessionMap -> SessionMap) -> GHState -> GHState
modSession f x = x { ghsSession = f $ ghsSession x }

-- | Internal use only, not to be confused with 'setHeader'.
addHeader :: Monad mo => Header -> GGHandler sub master mo ()
addHeader = GHandler . lift . lift . tell . (:)

getStatus :: ErrorResponse -> H.Status
getStatus NotFound = H.status404
getStatus (InternalError _) = H.status500
getStatus (InvalidArgs _) = H.status400
getStatus (PermissionDenied _) = H.status403
getStatus (BadMethod _) = H.status405

getRedirectStatus :: RedirectType -> H.Status
getRedirectStatus RedirectPermanent = H.status301
getRedirectStatus RedirectTemporary = H.status302
getRedirectStatus RedirectSeeOther = H.status303

-- | Different types of redirects.
data RedirectType = RedirectPermanent
                  | RedirectTemporary
                  | RedirectSeeOther
    deriving (Show, Eq)

localNoCurrent :: Monad mo => GGHandler s m mo a -> GGHandler s m mo a
localNoCurrent =
    GHandler . local (\hd -> hd { handlerRoute = Nothing }) . unGHandler

-- | Lookup for session data.
lookupSession :: Monad mo => Text -> GGHandler s m mo (Maybe Text)
lookupSession n = GHandler $ do
    m <- liftM ghsSession $ lift $ lift $ lift get
    return $ Map.lookup n m

-- | Get all session variables.
getSession :: Monad mo => GGHandler s m mo SessionMap
getSession = liftM ghsSession $ GHandler $ lift $ lift $ lift get

handlerToYAR :: (HasReps a, HasReps b)
             => m -- ^ master site foundation
             -> s -- ^ sub site foundation
             -> (Route s -> Route m)
             -> (Route m -> [(Text, Text)] -> Text)
             -> (ErrorResponse -> GHandler s m a)
             -> Request
             -> Maybe (Route s)
             -> SessionMap
             -> GHandler s m b
             -> Iteratee ByteString IO YesodAppResult
handlerToYAR y s toMasterRoute render errorHandler rr murl sessionMap h =
    unYesodApp ya eh' rr types sessionMap
  where
    ya = runHandler h render murl toMasterRoute y s
    eh' er = runHandler (errorHandler' er) render murl toMasterRoute y s
    types = httpAccept $ reqWaiRequest rr
    errorHandler' = localNoCurrent . errorHandler

type HeaderRenderer = [Header]
                   -> ContentType
                   -> SessionMap
                   -> [(CI H.Ascii, H.Ascii)]

yarToResponse :: HeaderRenderer -> YesodAppResult -> W.Response
yarToResponse _ (YARWai a) = a
yarToResponse renderHeaders (YARPlain s hs ct c sessionFinal) =
    case c of
        ContentBuilder b mlen ->
            let hs' = maybe finalHeaders finalHeaders' mlen
             in W.ResponseBuilder s hs' b
        ContentFile fp p -> W.ResponseFile s finalHeaders fp p
        ContentEnum e ->
            W.ResponseEnumerator $ \iter -> run_ $ e $$ iter s finalHeaders
  where
    finalHeaders = renderHeaders hs ct sessionFinal
    finalHeaders' len = ("Content-Length", S8.pack $ show len)
                      : finalHeaders
    {-
    getExpires m = fromIntegral (m * 60) `addUTCTime` now
    sessionVal =
        case key' of
            Nothing -> B.empty
            Just key'' -> encodeSession key'' exp' host
                        $ Map.toList
                        $ Map.insert nonceKey (reqNonce rr) sessionFinal
    hs' =
            case key' of
                Nothing -> hs
                Just _ -> AddCookie
                        (clientSessionDuration y)
                        sessionName
                        (bsToChars sessionVal)
                      : hs
    hs'' = map (headerToPair getExpires) hs'
    hs''' = ("Content-Type", charsToBs ct) : hs''
    -}

httpAccept :: W.Request -> [ContentType]
httpAccept = parseHttpAccept
           . fromMaybe mempty
           . lookup "Accept"
           . W.requestHeaders

-- | Convert Header to a key/value pair.
headerToPair :: S.ByteString -- ^ cookie path
             -> (Int -> UTCTime) -- ^ minutes -> expiration time
             -> Header
             -> (CI H.Ascii, H.Ascii)
headerToPair cp getExpires (AddCookie minutes key value) =
    ("Set-Cookie", toByteString $ renderSetCookie $ SetCookie
        { setCookieName = key
        , setCookieValue = value
        , setCookiePath = Just cp
        , setCookieExpires = Just $ getExpires minutes
        , setCookieDomain = Nothing
        })
headerToPair cp _ (DeleteCookie key) =
    ( "Set-Cookie"
    , key `mappend` "=; path=" `mappend` cp `mappend` "; expires=Thu, 01-Jan-1970 00:00:00 GMT"
    )
headerToPair _ _ (Header key value) = (key, value)

-- | Get a unique identifier.
newIdent :: Monad mo => GGHandler sub master mo String
newIdent = GHandler $ lift $ lift $ lift $ do
    x <- get
    let i' = ghsIdent x + 1
    put x { ghsIdent = i' }
    return $ "h" ++ show i'

liftIOHandler :: MonadIO mo
              => GGHandler sub master IO a
              -> GGHandler sub master mo a
liftIOHandler m = GHandler $
                   ReaderT $ \r ->
                     ErrorT $
                       WriterT $
                         StateT $ \s ->
                           liftIO $ runGGHandler m r s

runGGHandler :: GGHandler sub master m a
            -> HandlerData sub master
            -> GHState
            -> m ( ( Either HandlerContents a
                   , Endo [Header]
                   )
                 , GHState
                 )
runGGHandler m r s = runStateT
                      (runWriterT
                        (runErrorT
                          (runReaderT
                            (unGHandler m) r))) s

instance MonadTransControl (GGHandler s m) where
    liftControl f =
      GHandler $
        liftControl $ \runRdr ->
          liftControl $ \runErr ->
            liftControl $ \runWrt ->
              liftControl $ \runSt ->
                f ( liftM ( GHandler
                          . join . lift
                          . join . lift
                          . join . lift
                          )
                  . runSt . runWrt . runErr . runRdr
                  . unGHandler
                  )

-- | Redirect to a POST resource.
--
-- This is not technically a redirect; instead, it returns an HTML page with a
-- POST form, and some Javascript to automatically submit the form. This can be
-- useful when you need to post a plain link somewhere that needs to cause
-- changes on the server.
redirectToPost :: Monad mo => Route master -> GGHandler sub master mo a
redirectToPost dest = hamletToRepHtml
#if GHC7
            [hamlet|
#else
            [$hamlet|
#endif
\<!DOCTYPE html>

<html>
    <head>
        <title>Redirecting...
    <body onload="document.getElementById('form').submit()">
        <form id="form" method="post" action="@{dest}">
            <noscript>
                <p>Javascript has been disabled; please click on the button below to be redirected.
            <input type="submit" value="Continue">
|] >>= sendResponse

-- | Converts the given Hamlet template into 'Content', which can be used in a
-- Yesod 'Response'.
hamletToContent :: Monad mo
                => Hamlet (Route master) -> GGHandler sub master mo Content
hamletToContent h = do
    render <- getUrlRenderParams
    return $ toContent $ h render

-- | Wraps the 'Content' generated by 'hamletToContent' in a 'RepHtml'.
hamletToRepHtml :: Monad mo
                => Hamlet (Route master) -> GGHandler sub master mo RepHtml
hamletToRepHtml = liftM RepHtml . hamletToContent

-- | Get the request\'s 'W.Request' value.
waiRequest :: Monad mo => GGHandler sub master mo W.Request
waiRequest = reqWaiRequest `liftM` getRequest