servant/src/Servant/Server/Internal.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Servant.Server.Internal where

import Control.Applicative ((<$>))
import Control.Monad.Trans.Either (EitherT, runEitherT)
import Data.Aeson (ToJSON, FromJSON, encode, decode')
import qualified Data.ByteString as B
import qualified Data.ByteString.Lazy as BL
import Data.IORef (newIORef, readIORef, writeIORef)
import Data.List (unfoldr)
import Data.Maybe (catMaybes)
import Data.Monoid (Monoid, mempty, mappend)
import Data.Proxy (Proxy(Proxy))
import Data.String (fromString)
import Data.String.Conversions (cs, (<>))
import Data.Text.Encoding (decodeUtf8, encodeUtf8)
import Data.Text (Text)
import qualified Data.Text as T
import GHC.TypeLits (KnownSymbol, symbolVal)
import Network.HTTP.Types hiding (Header)
import Network.Wai (Response, Request, ResponseReceived, Application, pathInfo, requestBody,
                    strictRequestBody, lazyRequestBody, requestHeaders, requestMethod,
                    rawQueryString, responseLBS)
import Servant.API (QueryParams, QueryParam, QueryFlag, MatrixParams, MatrixParam, MatrixFlag, ReqBody, Header, Capture, Get, Delete, Put, Post, Raw, (:>), (:<|>)(..))
import Servant.Common.Text (FromText, fromText)

data ReqBodyState = Uncalled
                  | Called !B.ByteString
                  | Done !B.ByteString

toApplication :: RoutingApplication -> Application
toApplication ra request respond = do
  reqBodyRef <- newIORef Uncalled
  -- We may need to consume the requestBody more than once.  In order to
  -- maintain the illusion that 'requestBody' works as expected,
  -- 'ReqBodyState' is introduced, and the complete body is memoized and
  -- returned as many times as requested with empty "Done" marker chunks in
  -- between.
  -- See https://github.com/haskell-servant/servant/issues/3
  let memoReqBody = do
          ior <- readIORef reqBodyRef
          case ior of
            Uncalled -> do
                r <- BL.toStrict <$> strictRequestBody request
                writeIORef reqBodyRef $ Done r
                return r
            Called bs -> do
                writeIORef reqBodyRef $ Done bs
                return bs
            Done bs -> do
                writeIORef reqBodyRef $ Called bs
                return B.empty

  ra request{ requestBody = memoReqBody } (routingRespond . routeResult)
 where
  routingRespond :: Either RouteMismatch Response -> IO ResponseReceived
  routingRespond (Left NotFound) =
    respond $ responseLBS notFound404 [] "not found"
  routingRespond (Left WrongMethod) =
    respond $ responseLBS methodNotAllowed405 [] "method not allowed"
  routingRespond (Left InvalidBody) =
    respond $ responseLBS badRequest400 [] "Invalid JSON in request body"
  routingRespond (Right response) =
    respond response

-- * Route mismatch
data RouteMismatch =
    NotFound    -- ^ the usual "not found" error
  | WrongMethod -- ^ a more informative "you just got the HTTP method wrong" error
  | InvalidBody -- ^ an even more informative "your json request body wasn't valid" error
  deriving (Eq, Show)

-- |
-- @
-- > mempty = NotFound
-- >
-- > NotFound    `mappend`           x = x
-- > WrongMethod `mappend` InvalidBody = InvalidBody
-- > WrongMethod `mappend`           _ = WrongMethod
-- > InvalidBody `mappend`           _ = InvalidBody
-- @
instance Monoid RouteMismatch where
  mempty = NotFound

  NotFound    `mappend`           x = x
  WrongMethod `mappend` InvalidBody = InvalidBody
  WrongMethod `mappend`           _ = WrongMethod
  InvalidBody `mappend`           _ = InvalidBody

-- | A wrapper around @'Either' 'RouteMismatch' a@.
newtype RouteResult a =
  RR { routeResult :: Either RouteMismatch a }
  deriving (Eq, Show)

failWith :: RouteMismatch -> RouteResult a
failWith = RR . Left

succeedWith :: a -> RouteResult a
succeedWith = RR . Right

isMismatch :: RouteResult a -> Bool
isMismatch (RR (Left _)) = True
isMismatch _             = False

-- | Like `null . pathInfo`, but works with redundant trailing slashes.
pathIsEmpty :: Request -> Bool
pathIsEmpty = f . processedPathInfo
  where
    f []   = True
    f [""] = True
    f _    = False

-- | If we get a `Right`, it has precedence over everything else.
--
-- This in particular means that if we could get several 'Right's,
-- only the first we encounter would be taken into account.
instance Monoid (RouteResult a) where
  mempty = RR $ Left mempty

  RR (Left x)  `mappend` RR (Left y)  = RR $ Left (x <> y)
  RR (Left _)  `mappend` RR (Right y) = RR $ Right y
  r            `mappend` _            = r

type RoutingApplication =
     Request -- ^ the request, the field 'pathInfo' may be modified by url routing
  -> (RouteResult Response -> IO ResponseReceived) -> IO ResponseReceived

splitMatrixParameters :: Text -> (Text, Text)
splitMatrixParameters = T.break (== ';')

parsePathInfo :: Request -> [Text]
parsePathInfo = filter (/= "") . mergePairs . map splitMatrixParameters . pathInfo
  where mergePairs = concat . unfoldr pairToList
        pairToList []          = Nothing
        pairToList ((a, b):xs) = Just ([a, b], xs)

-- | Returns a processed pathInfo from the request.
--
-- In order to handle matrix parameters in the request correctly, the raw pathInfo needs to be
-- processed, so routing works as intended. Therefor this function should be used to access
-- the pathInfo for routing purposes.
processedPathInfo :: Request -> [Text]
processedPathInfo r =
  case pinfo of
    (x:xs) | T.head x == ';' -> xs
    _                        -> pinfo
  where pinfo = parsePathInfo r

class HasServer layout where
  type Server layout :: *
  route :: Proxy layout -> Server layout -> RoutingApplication



-- * Instances

-- | A server for @a ':<|>' b@ first tries to match the request against the route
--   represented by @a@ and if it fails tries @b@. You must provide a request
--   handler for each route.
--
-- > type MyApi = "books" :> Get [Book] -- GET /books
-- >         :<|> "books" :> ReqBody Book :> Post Book -- POST /books
-- >
-- > server :: Server MyApi
-- > server = listAllBooks :<|> postBook
-- >   where listAllBooks = ...
-- >         postBook book = ...
instance (HasServer a, HasServer b) => HasServer (a :<|> b) where
  type Server (a :<|> b) = Server a :<|> Server b
  route Proxy (a :<|> b) request respond =
    route pa a request $ \ mResponse ->
      if isMismatch mResponse
        then route pb b request $ \mResponse' -> respond (mResponse <> mResponse')
        else respond mResponse

    where pa = Proxy :: Proxy a
          pb = Proxy :: Proxy b

captured :: FromText a => proxy (Capture sym a) -> Text -> Maybe a
captured _ = fromText

-- | If you use 'Capture' in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of the type specified by the 'Capture'.
-- This lets servant worry about getting it from the URL and turning
-- it into a value of the type you specify.
--
-- You can control how it'll be converted from 'Text' to your type
-- by simply providing an instance of 'FromText' for your type.
--
-- Example:
--
-- > type MyApi = "books" :> Capture "isbn" Text :> Get Book
-- >
-- > server :: Server MyApi
-- > server = getBook
-- >   where getBook :: Text -> EitherT (Int, String) IO Book
-- >         getBook isbn = ...
instance (KnownSymbol capture, FromText a, HasServer sublayout)
      => HasServer (Capture capture a :> sublayout) where

  type Server (Capture capture a :> sublayout) =
     a -> Server sublayout

  route Proxy subserver request respond = case processedPathInfo request of
    (first : rest)
      -> case captured captureProxy first of
           Nothing  -> respond $ failWith NotFound
           Just v   -> route (Proxy :: Proxy sublayout) (subserver v) request{
                         pathInfo = rest
                       } respond
    _ -> respond $ failWith NotFound

    where captureProxy = Proxy :: Proxy (Capture capture a)
           

-- | If you have a 'Delete' endpoint in your API,
-- the handler for this endpoint is meant to delete
-- a resource.
--
-- The code of the handler will, just like
-- for 'Servant.API.Get.Get', 'Servant.API.Post.Post' and
-- 'Servant.API.Put.Put', run in @EitherT (Int, String) IO ()@.
-- The 'Int' represents the status code and the 'String' a message
-- to be returned. You can use 'Control.Monad.Trans.Either.left' to
-- painlessly error out if the conditions for a successful deletion
-- are not met.
instance HasServer Delete where
  type Server Delete = EitherT (Int, String) IO ()

  route Proxy action request respond
    | pathIsEmpty request && requestMethod request == methodDelete = do
        e <- runEitherT action
        respond $ succeedWith $ case e of
          Right () ->
            responseLBS status204 [] ""
          Left (status, message) ->
            responseLBS (mkStatus status (cs message)) [] (cs message)
    | pathIsEmpty request && requestMethod request /= methodDelete =
        respond $ failWith WrongMethod
    | otherwise = respond $ failWith NotFound

-- | When implementing the handler for a 'Get' endpoint,
-- just like for 'Servant.API.Delete.Delete', 'Servant.API.Post.Post'
-- and 'Servant.API.Put.Put', the handler code runs in the
-- @EitherT (Int, String) IO@ monad, where the 'Int' represents
-- the status code and the 'String' a message, returned in case of
-- failure. You can quite handily use 'Control.Monad.Trans.EitherT.left'
-- to quickly fail if some conditions are not met.
--
-- If successfully returning a value, we just require that its type has
-- a 'ToJSON' instance and servant takes care of encoding it for you,
-- yielding status code 200 along the way.
instance ToJSON result => HasServer (Get result) where
  type Server (Get result) = EitherT (Int, String) IO result
  route Proxy action request respond
    | pathIsEmpty request && requestMethod request == methodGet = do
        e <- runEitherT action
        respond . succeedWith $ case e of
          Right output ->
            responseLBS ok200 [("Content-Type", "application/json")] (encode output)
          Left (status, message) ->
            responseLBS (mkStatus status (cs message)) [] (cs message)
    | pathIsEmpty request && requestMethod request /= methodGet =
        respond $ failWith WrongMethod
    | otherwise = respond $ failWith NotFound

-- | If you use 'Header' in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of the type specified by 'Header'.
-- This lets servant worry about extracting it from the request and turning
-- it into a value of the type you specify.
--
-- All it asks is for a 'FromText' instance.
--
-- Example:
--
-- > newtype Referer = Referer Text
-- >   deriving (Eq, Show, FromText, ToText)
-- >
-- >            -- GET /view-my-referer
-- > type MyApi = "view-my-referer" :> Header "Referer" Referer :> Get Referer
-- >
-- > server :: Server MyApi
-- > server = viewReferer
-- >   where viewReferer :: Referer -> EitherT (Int, String) IO referer
-- >         viewReferer referer = return referer
instance (KnownSymbol sym, FromText a, HasServer sublayout)
      => HasServer (Header sym a :> sublayout) where

  type Server (Header sym a :> sublayout) =
    Maybe a -> Server sublayout

  route Proxy subserver request respond = do
    let mheader = fromText . decodeUtf8 =<< lookup str (requestHeaders request)
    route (Proxy :: Proxy sublayout) (subserver mheader) request respond

      where str = fromString $ symbolVal (Proxy :: Proxy sym)

-- | When implementing the handler for a 'Post' endpoint,
-- just like for 'Servant.API.Delete.Delete', 'Servant.API.Get.Get'
-- and 'Servant.API.Put.Put', the handler code runs in the
-- @EitherT (Int, String) IO@ monad, where the 'Int' represents
-- the status code and the 'String' a message, returned in case of
-- failure. You can quite handily use 'Control.Monad.Trans.EitherT.left'
-- to quickly fail if some conditions are not met.
--
-- If successfully returning a value, we just require that its type has
-- a 'ToJSON' instance and servant takes care of encoding it for you,
-- yielding status code 201 along the way.
instance ToJSON a => HasServer (Post a) where
  type Server (Post a) = EitherT (Int, String) IO a

  route Proxy action request respond
    | pathIsEmpty request && requestMethod request == methodPost = do
        e <- runEitherT action
        respond . succeedWith $ case e of
          Right out ->
            responseLBS status201 [("Content-Type", "application/json")] (encode out)
          Left (status, message) ->
            responseLBS (mkStatus status (cs message)) [] (cs message)
    | pathIsEmpty request && requestMethod request /= methodPost =
        respond $ failWith WrongMethod
    | otherwise = respond $ failWith NotFound

-- | When implementing the handler for a 'Put' endpoint,
-- just like for 'Servant.API.Delete.Delete', 'Servant.API.Get.Get'
-- and 'Servant.API.Post.Post', the handler code runs in the
-- @EitherT (Int, String) IO@ monad, where the 'Int' represents
-- the status code and the 'String' a message, returned in case of
-- failure. You can quite handily use 'Control.Monad.Trans.EitherT.left'
-- to quickly fail if some conditions are not met.
--
-- If successfully returning a value, we just require that its type has
-- a 'ToJSON' instance and servant takes care of encoding it for you,
-- yielding status code 200 along the way.
instance ToJSON a => HasServer (Put a) where
  type Server (Put a) = EitherT (Int, String) IO a

  route Proxy action request respond
    | pathIsEmpty request && requestMethod request == methodPut = do
        e <- runEitherT action
        respond . succeedWith $ case e of
          Right out ->
            responseLBS ok200 [("Content-Type", "application/json")] (encode out)
          Left (status, message) ->
            responseLBS (mkStatus status (cs message)) [] (cs message)
    | pathIsEmpty request && requestMethod request /= methodPut =
        respond $ failWith WrongMethod

    | otherwise = respond $ failWith NotFound

-- | If you use @'QueryParam' "author" Text@ in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of type @'Maybe' 'Text'@.
--
-- This lets servant worry about looking it up in the query string
-- and turning it into a value of the type you specify, enclosed
-- in 'Maybe', because it may not be there and servant would then
-- hand you 'Nothing'.
--
-- You can control how it'll be converted from 'Text' to your type
-- by simply providing an instance of 'FromText' for your type.
--
-- Example:
--
-- > type MyApi = "books" :> QueryParam "author" Text :> Get [Book]
-- >
-- > server :: Server MyApi
-- > server = getBooksBy
-- >   where getBooksBy :: Maybe Text -> EitherT (Int, String) IO [Book]
-- >         getBooksBy Nothing       = ...return all books...
-- >         getBooksBy (Just author) = ...return books by the given author...
instance (KnownSymbol sym, FromText a, HasServer sublayout)
      => HasServer (QueryParam sym a :> sublayout) where

  type Server (QueryParam sym a :> sublayout) =
    Maybe a -> Server sublayout

  route Proxy subserver request respond = do
    let querytext = parseQueryText $ rawQueryString request
        param =
          case lookup paramname querytext of
            Nothing       -> Nothing -- param absent from the query string
            Just Nothing  -> Nothing -- param present with no value -> Nothing
            Just (Just v) -> fromText v -- if present, we try to convert to
                                        -- the right type

    route (Proxy :: Proxy sublayout) (subserver param) request respond

    where paramname = cs $ symbolVal (Proxy :: Proxy sym)

-- | If you use @'QueryParams' "authors" Text@ in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of type @['Text']@.
--
-- This lets servant worry about looking up 0 or more values in the query string
-- associated to @authors@ and turning each of them into a value of
-- the type you specify.
--
-- You can control how the individual values are converted from 'Text' to your type
-- by simply providing an instance of 'FromText' for your type.
--
-- Example:
--
-- > type MyApi = "books" :> QueryParams "authors" Text :> Get [Book]
-- >
-- > server :: Server MyApi
-- > server = getBooksBy
-- >   where getBooksBy :: [Text] -> EitherT (Int, String) IO [Book]
-- >         getBooksBy authors = ...return all books by these authors...
instance (KnownSymbol sym, FromText a, HasServer sublayout)
      => HasServer (QueryParams sym a :> sublayout) where

  type Server (QueryParams sym a :> sublayout) =
    [a] -> Server sublayout

  route Proxy subserver request respond = do
    let querytext = parseQueryText $ rawQueryString request
        -- if sym is "foo", we look for query string parameters
        -- named "foo" or "foo[]" and call fromText on the
        -- corresponding values
        parameters = filter looksLikeParam querytext
        values = catMaybes $ map (convert . snd) parameters

    route (Proxy :: Proxy sublayout) (subserver values) request respond

    where paramname = cs $ symbolVal (Proxy :: Proxy sym)
          looksLikeParam (name, _) = name == paramname || name == (paramname <> "[]")
          convert Nothing = Nothing
          convert (Just v) = fromText v

-- | If you use @'QueryFlag' "published"@ in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of type 'Bool'.
--
-- Example:
--
-- > type MyApi = "books" :> QueryFlag "published" :> Get [Book]
-- >
-- > server :: Server MyApi
-- > server = getBooks
-- >   where getBooks :: Bool -> EitherT (Int, String) IO [Book]
-- >         getBooks onlyPublished = ...return all books, or only the ones that are already published, depending on the argument...
instance (KnownSymbol sym, HasServer sublayout)
      => HasServer (QueryFlag sym :> sublayout) where

  type Server (QueryFlag sym :> sublayout) =
    Bool -> Server sublayout

  route Proxy subserver request respond = do
    let querytext = parseQueryText $ rawQueryString request
        param = case lookup paramname querytext of
          Just Nothing  -> True  -- param is there, with no value
          Just (Just v) -> examine v -- param with a value
          Nothing       -> False -- param not in the query string

    route (Proxy :: Proxy sublayout) (subserver param) request respond

    where paramname = cs $ symbolVal (Proxy :: Proxy sym)
          examine v | v == "true" || v == "1" || v == "" = True
                    | otherwise = False

parseMatrixText :: B.ByteString -> QueryText
parseMatrixText = parseQueryText

-- | If you use @'MatrixParam' "author" Text@ in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of type @'Maybe' 'Text'@.
--
-- This lets servant worry about looking it up in the query string
-- and turning it into a value of the type you specify, enclosed
-- in 'Maybe', because it may not be there and servant would then
-- hand you 'Nothing'.
--
-- You can control how it'll be converted from 'Text' to your type
-- by simply providing an instance of 'FromText' for your type.
--
-- Example:
--
-- > type MyApi = "books" :> MatrixParam "author" Text :> Get [Book]
-- >
-- > server :: Server MyApi
-- > server = getBooksBy
-- >   where getBooksBy :: Maybe Text -> EitherT (Int, String) IO [Book]
-- >         getBooksBy Nothing       = ...return all books...
-- >         getBooksBy (Just author) = ...return books by the given author...
instance (KnownSymbol sym, FromText a, HasServer sublayout)
      => HasServer (MatrixParam sym a :> sublayout) where

  type Server (MatrixParam sym a :> sublayout) =
    Maybe a -> Server sublayout

  route Proxy subserver request respond = case parsePathInfo request of
    (first : _)
      -> do let querytext = parseMatrixText . encodeUtf8 $ T.tail first
                param = case lookup paramname querytext of
                  Nothing       -> Nothing -- param absent from the query string
                  Just Nothing  -> Nothing -- param present with no value -> Nothing
                  Just (Just v) -> fromText v -- if present, we try to convert to
                                        -- the right type
            route (Proxy :: Proxy sublayout) (subserver param) request respond
    _ -> route (Proxy :: Proxy sublayout) (subserver Nothing) request respond

    where paramname = cs $ symbolVal (Proxy :: Proxy sym)

-- | If you use @'MatrixParams' "authors" Text@ in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of type @['Text']@.
--
-- This lets servant worry about looking up 0 or more values in the query string
-- associated to @authors@ and turning each of them into a value of
-- the type you specify.
--
-- You can control how the individual values are converted from 'Text' to your type
-- by simply providing an instance of 'FromText' for your type.
--
-- Example:
--
-- > type MyApi = "books" :> MatrixParams "authors" Text :> Get [Book]
-- >
-- > server :: Server MyApi
-- > server = getBooksBy
-- >   where getBooksBy :: [Text] -> EitherT (Int, String) IO [Book]
-- >         getBooksBy authors = ...return all books by these authors...
instance (KnownSymbol sym, FromText a, HasServer sublayout)
      => HasServer (MatrixParams sym a :> sublayout) where

  type Server (MatrixParams sym a :> sublayout) =
    [a] -> Server sublayout

  route Proxy subserver request respond = case parsePathInfo request of
    (first : _)
      -> do let matrixtext = parseMatrixText . encodeUtf8 $ T.tail first
                -- if sym is "foo", we look for matrix parameters
                -- named "foo" or "foo[]" and call fromText on the
                -- corresponding values
                parameters = filter looksLikeParam matrixtext
                values = catMaybes $ map (convert . snd) parameters
            route (Proxy :: Proxy sublayout) (subserver values) request respond
    _ -> route (Proxy :: Proxy sublayout) (subserver []) request respond

    where paramname = cs $ symbolVal (Proxy :: Proxy sym)
          looksLikeParam (name, _) = name == paramname || name == (paramname <> "[]")
          convert Nothing = Nothing
          convert (Just v) = fromText v

-- | If you use @'MatrixFlag' "published"@ in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of type 'Bool'.
--
-- Example:
--
-- > type MyApi = "books" :> MatrixFlag "published" :> Get [Book]
-- >
-- > server :: Server MyApi
-- > server = getBooks
-- >   where getBooks :: Bool -> EitherT (Int, String) IO [Book]
-- >         getBooks onlyPublished = ...return all books, or only the ones that are already published, depending on the argument...
instance (KnownSymbol sym, HasServer sublayout)
      => HasServer (MatrixFlag sym :> sublayout) where

  type Server (MatrixFlag sym :> sublayout) =
    Bool -> Server sublayout

  route Proxy subserver request respond =  case parsePathInfo request of
    (first : _)
      -> do let matrixtext = parseMatrixText . encodeUtf8 $ T.tail first
                param = case lookup paramname matrixtext of
                  Just Nothing  -> True  -- param is there, with no value
                  Just (Just v) -> examine v -- param with a value
                  Nothing       -> False -- param not in the query string

            route (Proxy :: Proxy sublayout) (subserver param) request respond

    _ -> route (Proxy :: Proxy sublayout) (subserver False) request respond

    where paramname = cs $ symbolVal (Proxy :: Proxy sym)
          examine v | v == "true" || v == "1" || v == "" = True
                    | otherwise = False

-- | Just pass the request to the underlying application and serve its response.
--
-- Example:
--
-- > type MyApi = "images" :> Raw
-- >
-- > server :: Server MyApi
-- > server = serveDirectory "/var/www/images"
instance HasServer Raw where
  type Server Raw = Application
  route Proxy rawApplication request respond =
    rawApplication request (respond . succeedWith)

-- | If you use 'ReqBody' in one of the endpoints for your API,
-- this automatically requires your server-side handler to be a function
-- that takes an argument of the type specified by 'ReqBody'.
-- This lets servant worry about extracting it from the request and turning
-- it into a value of the type you specify.
--
-- All it asks is for a 'FromJSON' instance.
--
-- Example:
--
-- > type MyApi = "books" :> ReqBody Book :> Post Book
-- >
-- > server :: Server MyApi
-- > server = postBook
-- >   where postBook :: Book -> EitherT (Int, String) IO Book
-- >         postBook book = ...insert into your db...
instance (FromJSON a, HasServer sublayout)
      => HasServer (ReqBody a :> sublayout) where

  type Server (ReqBody a :> sublayout) =
    a -> Server sublayout

  route Proxy subserver request respond = do
    mrqbody <- decode' <$> lazyRequestBody request
    case mrqbody of
      Nothing -> respond $ failWith InvalidBody
      Just v  -> route (Proxy :: Proxy sublayout) (subserver v) request respond

-- | Make sure the incoming request starts with @"/path"@, strip it and
-- pass the rest of the request path to @sublayout@.
instance (KnownSymbol path, HasServer sublayout) => HasServer (path :> sublayout) where
  type Server (path :> sublayout) = Server sublayout
  route Proxy subserver request respond = case processedPathInfo request of
    (first : rest)
      | first == cs (symbolVal proxyPath)
      -> route (Proxy :: Proxy sublayout) subserver request{
           pathInfo = rest
         } respond
    _ -> respond $ failWith NotFound

    where proxyPath = Proxy :: Proxy path