    {-# LANGUAGE GADTs #-}
    {-# LANGUAGE LambdaCase #-}
    {-# LANGUAGE OverloadedStrings #-}
    {-# LANGUAGE PatternGuards #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE UnicodeSyntax #-}

    -- |
    -- Module: Network.Wai.Middleware.Cors
    -- Description: Cross-Origin resource sharing (CORS) for WAI
    -- Copyright:
    --     © 2015-2019 Lars Kuhtz <lakuhtz@gmail.com,
    --     © 2014 AlephCloud Systems, Inc.
    -- License: MIT
    -- Maintainer: Lars Kuhtz <lakuhtz@gmail.com>
    -- Stability: stable
    --
    -- An implemenation of Cross-Origin resource sharing (CORS) for WAI that
    -- aims to be compliant with <http://www.w3.org/TR/cors>.
    --
    -- The function 'simpleCors' enables support of simple cross-origin requests. More
    -- advanced CORS policies can be enabled by passing a 'CorsResourcePolicy' to the
    -- 'cors' middleware.
    --
    -- = Note On Security
    --
    -- This implementation doens't include any server side enforcement. By
    -- complying with the CORS standard it enables the client (i.e. the web
    -- browser) to enforce the CORS policy. For application authors it is strongly
    -- recommended to take into account the security considerations in section 6.3
    -- of <http://www.w3.org/TR/cors>. In particular the application should check
    -- that the value of the @Origin@ header matches it's expectations.
    --
    -- = Websockets
    --
    -- Websocket connections don't support CORS and are ignored by this CORS
    -- implementation. However Websocket requests usually (at least for some
    -- browsers) include the @Origin@ header. Applications are expected to check
    -- the value of this header and respond with an error in case that its content
    -- doesn't match the expectations.
    --
    -- = Example
    --
    -- The following is an example how to enable support for simple cross-origin requests
    -- for a <http://hackage.haskell.org/package/scotty scotty> application.
    --
    -- > {-# LANGUAGE UnicodeSyntax #-}
    -- > {-# LANGUAGE OverloadedStrings #-}
    -- >
    -- > module Main
    -- > ( main
    -- > ) where
    -- >
    -- > import Network.Wai.Middleware.Cors
    -- > import Web.Scotty
    -- >
    -- > main ∷ IO ()
    -- > main = scotty 8080 $ do
    -- >     middleware simpleCors
    -- >     matchAny  "/" $ text "Success"
    --
    -- The result of following curl command will include the HTTP response header
    -- @Access-Control-Allow-Origin: *@.
    --
    -- > curl -i http://127.0.0.1:8888 -H 'Origin: 127.0.0.1' -v
    --
    module Network.Wai.Middleware.Cors
    ( Origin
    , CorsResourcePolicy(..)
    , simpleCorsResourcePolicy
    , cors
    , simpleCors

    -- * Utils
    , isSimple
    , simpleResponseHeaders
    , simpleHeaders
    , simpleContentTypes
    , simpleMethods
    ) where

    import Control.Monad.Error.Class
    import Control.Monad.Trans.Except

    import qualified Data.Attoparsec.ByteString.Char8 as P
    import qualified Data.ByteString.Char8 as B8
    import qualified Data.ByteString.Lazy.Char8 as LB8
    import qualified Data.CaseInsensitive as CI
    import Data.List (intersect, (\\), union)
    import Data.Maybe (catMaybes)
    import Data.Monoid.Unicode
    import Data.String

    import qualified Network.HTTP.Types as HTTP
    import qualified Network.Wai as WAI

    import Prelude.Unicode

    -- | Origins are expected to be formated as described in
    -- <https://www.ietf.org/rfc/rfc6454.txt RFC6454> (section 6.2).
    -- In particular the string @*@ is not a valid origin (but the string
    -- @null@ is).
    --
    type Origin = B8.ByteString

    data CorsResourcePolicy = CorsResourcePolicy
        {

        -- | HTTP origins that are allowed in CORS requests.
        --
        -- A value of 'Nothing' indicates unrestricted cross-origin sharing and
        -- results in @*@ as value for the @Access-Control-Allow-Origin@ HTTP
        -- response header. Note if you send @*@, credentials cannot be sent with the request.
        --
        -- A value other than 'Nothing' is a tuple that consists of a list of
        -- origins and a Boolean flag that indicates if credentials are used
        -- to access the resource via CORS.
        --
        -- Origins must be formated as described in
        -- <https://www.ietf.org/rfc/rfc6454.txt RFC6454> (section 6.2). In
        -- particular the string @*@ is not a valid origin (but the string @null@
        -- is).
        --
        -- Credentials include cookies, authorization headers and TLS client certificates.
        -- For credentials to be sent with requests, the @withCredentials@ setting of
        -- @XmlHttpRequest@ in the browser must be set to @true@.
        --
           CorsResourcePolicy -> Maybe ([Origin], Bool)
    corsOrigins ∷ !(Maybe ([Origin], Bool))

        -- | HTTP methods that are allowed in CORS requests.
        --
        , CorsResourcePolicy -> [Origin]
    corsMethods ∷ ![HTTP.Method]

        -- | Field names of HTTP request headers that are allowed in CORS requests.
        -- Header names that are included in 'simpleHeaders', except for
        -- @content-type@, are implicitly included and thus optional in this list.
        --
        , CorsResourcePolicy -> [HeaderName]
    corsRequestHeaders ∷ ![HTTP.HeaderName]

        -- | Field names of HTTP headers that are exposed to the client in the response.
        --
        , CorsResourcePolicy -> Maybe [HeaderName]
    corsExposedHeaders ∷ !(Maybe [HTTP.HeaderName])

        -- | Number of seconds that the OPTIONS preflight response may be cached by the client.
        --
        -- Tip: Set this to 'Nothing' while testing your CORS implementation, then increase
        -- it once you deploy to production.
        --
        , CorsResourcePolicy -> Maybe Int
    corsMaxAge ∷ !(Maybe Int)

        -- | If the resource is shared by multiple origins but
        -- @Access-Control-Allow-Origin@ is not set to @*@ this may be set to
        -- 'True' to cause the server to include a @Vary: Origin@ header in the
        -- response, thus indicating that the value of the
        -- @Access-Control-Allow-Origin@ header may vary between different requests
        -- for the same resource. This prevents caching of the responses which may
        -- not apply accross different origins.
        --
        , CorsResourcePolicy -> Bool
    corsVaryOrigin ∷ !Bool

        -- | If this is 'True' and the request does not include an @Origin@ header
        -- the response has HTTP status 400 (bad request) and the body contains
        -- a short error message.
        --
        -- If this is 'False' and the request does not include an @Origin@ header
        -- the request is passed on unchanged to the application.
        --
        -- @since 0.2
        , CorsResourcePolicy -> Bool
    corsRequireOrigin ∷ !Bool

        -- | In the case that
        --
        -- * the request contains an @Origin@ header and
        --
        -- * the client does not conform with the CORS protocol
        --   (/request is out of scope/)
        --
        -- then
        --
        -- * the request is passed on unchanged to the application if this field is
        --   'True' or
        --
        -- * an response with HTTP status 400 (bad request) and short
        --   error message is returned if this field is 'False'.
        --
        -- Note: Your application needs to will receive preflight OPTIONS requests if set to 'True'.
        --
        -- @since 0.2
        --
        , CorsResourcePolicy -> Bool
    corsIgnoreFailures ∷ !Bool
        }
        deriving (Int -> CorsResourcePolicy -> ShowS
    [CorsResourcePolicy] -> ShowS
    CorsResourcePolicy -> String
    (Int -> CorsResourcePolicy -> ShowS)
    -> (CorsResourcePolicy -> String)
    -> ([CorsResourcePolicy] -> ShowS)
    -> Show CorsResourcePolicy
    forall a.
    (Int -> a -> ShowS) -> (a -> String) -> ([a] -> ShowS) -> Show a
    showList :: [CorsResourcePolicy] -> ShowS
    $cshowList :: [CorsResourcePolicy] -> ShowS
    show :: CorsResourcePolicy -> String
    $cshow :: CorsResourcePolicy -> String
    showsPrec :: Int -> CorsResourcePolicy -> ShowS
    $cshowsPrec :: Int -> CorsResourcePolicy -> ShowS
    Show,ReadPrec [CorsResourcePolicy]
    ReadPrec CorsResourcePolicy
    Int -> ReadS CorsResourcePolicy
    ReadS [CorsResourcePolicy]
    (Int -> ReadS CorsResourcePolicy)
    -> ReadS [CorsResourcePolicy]
    -> ReadPrec CorsResourcePolicy
    -> ReadPrec [CorsResourcePolicy]
    -> Read CorsResourcePolicy
    forall a.
    (Int -> ReadS a)
    -> ReadS [a] -> ReadPrec a -> ReadPrec [a] -> Read a
    readListPrec :: ReadPrec [CorsResourcePolicy]
    $creadListPrec :: ReadPrec [CorsResourcePolicy]
    readPrec :: ReadPrec CorsResourcePolicy
    $creadPrec :: ReadPrec CorsResourcePolicy
    readList :: ReadS [CorsResourcePolicy]
    $creadList :: ReadS [CorsResourcePolicy]
    readsPrec :: Int -> ReadS CorsResourcePolicy
    $creadsPrec :: Int -> ReadS CorsResourcePolicy
    Read,CorsResourcePolicy -> CorsResourcePolicy -> Bool
    (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> Eq CorsResourcePolicy
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    /= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $c/= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    == :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $c== :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    Eq,Eq CorsResourcePolicy
    Eq CorsResourcePolicy =>
    (CorsResourcePolicy -> CorsResourcePolicy -> Ordering)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy)
    -> Ord CorsResourcePolicy
    CorsResourcePolicy -> CorsResourcePolicy -> Bool
    CorsResourcePolicy -> CorsResourcePolicy -> Ordering
    CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    forall a.
    Eq a =>
    (a -> a -> Ordering)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> Bool)
    -> (a -> a -> a)
    -> (a -> a -> a)
    -> Ord a
    min :: CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    $cmin :: CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    max :: CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    $cmax :: CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    >= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $c>= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    > :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $c> :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    <= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $c<= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    < :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $c< :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    compare :: CorsResourcePolicy -> CorsResourcePolicy -> Ordering
    $ccompare :: CorsResourcePolicy -> CorsResourcePolicy -> Ordering
    $cp1Ord :: Eq CorsResourcePolicy
    Ord)

    -- | A 'CorsResourcePolicy' that supports /simple cross-origin requests/ as defined
    -- in <http://www.w3.org/TR/cors/>.
    --
    -- * The HTTP header @Access-Control-Allow-Origin@ is set to @*@.
    --
    -- * Request methods are constraint to /simple methods/ (@GET@, @HEAD@, @POST@).
    --
    -- * Request headers are constraint to /simple request headers/
    --   (@Accept@, @Accept-Language@, @Content-Language@, @Content-Type@).
    --
    -- * If the request is a @POST@ request the content type is constraint to
    --    /simple content types/
    --    (@application/x-www-form-urlencoded@, @multipart/form-data@, @text/plain@),
    --
    -- * Only /simple response headers/ may be exposed on the client
    --   (@Cache-Control@, @Content-Language@, @Content-Type@, @Expires@, @Last-Modified@,  @Pragma@)
    --
    -- * The @Vary-Origin@ header is left unchanged (possibly unset).
    --
    -- * If the request doesn't include an @Origin@ header the request is passed unchanged to
    --   the application.
    --
    -- * If the request includes an @Origin@ header but does not conform to the CORS
    --   protocol (/request is out of scope/) an response with HTTP status 400 (bad request)
    --   and a short error message is returned.
    --
    -- For /simple cross-origin requests/ a preflight request is not required. However, if
    -- the client chooses to make a preflight request it is answered in accordance with
    -- the policy for /simple cross-origin requests/.
    --
    simpleCorsResourcePolicy ∷ CorsResourcePolicy
    simpleCorsResourcePolicy :: CorsResourcePolicy
    simpleCorsResourcePolicy = $WCorsResourcePolicy :: Maybe ([Origin], Bool)
    -> [Origin]
    -> [HeaderName]
    -> Maybe [HeaderName]
    -> Maybe Int
    -> Bool
    -> Bool
    -> Bool
    -> CorsResourcePolicy
    CorsResourcePolicy
        { corsOrigins :: Maybe ([Origin], Bool)
    corsOrigins = Maybe ([Origin], Bool)
    forall a. Maybe a
    Nothing
        , corsMethods :: [Origin]
    corsMethods = [Origin]
    simpleMethods
        , corsRequestHeaders :: [HeaderName]
    corsRequestHeaders = []
        , corsExposedHeaders :: Maybe [HeaderName]
    corsExposedHeaders = Maybe [HeaderName]
    forall a. Maybe a
    Nothing
        , corsMaxAge :: Maybe Int
    corsMaxAge = Maybe Int
    forall a. Maybe a
    Nothing
        , corsVaryOrigin :: Bool
    corsVaryOrigin = Bool
    False
        , corsRequireOrigin :: Bool
    corsRequireOrigin = Bool
    False
        , corsIgnoreFailures :: Bool
    corsIgnoreFailures = Bool
    False
        }

    -- | A Cross-Origin resource sharing (CORS) middleware.
    --
    -- The middleware is given a function that serves as a pattern to decide
    -- whether a requested resource is available for CORS. If the match fails with
    -- 'Nothing' the request is passed unmodified to the inner application.
    --
    -- The current version of this module does only aim at compliance with the CORS
    -- protocol as specified in <http://www.w3.org/TR/cors/>. In accordance with
    -- that standard the role of the server side is to support the client to
    -- enforce CORS restrictions. This module does not implement any enforcement of
    -- authorization policies that are possibly implied by the
    -- 'CorsResourcePolicy'. It is up to the inner WAI application to enforce such
    -- policy and make sure that it is in accordance with the configuration of the
    -- 'cors' middleware.
    --
    -- Matches are done as follows: @*@ matches every origin. For all other cases a
    -- match succeeds if and only if the ASCII serializations (as described in
    -- RCF6454 section 6.2) are equal.
    --
    -- The OPTIONS method may return options for resources that are not actually
    -- available. In particular for preflight requests the implementation returns
    -- for the HTTP response headers @Access-Control-Allow-Headers@ and
    -- @Access-Control-Allow-Methods@ all values specified in the
    -- 'CorsResourcePolicy' together with the respective values for simple requests
    -- (except @content-type@). This does not imply that the application actually
    -- supports the respective values are for the requested resource. Thus,
    -- depending on the application, an actual request may still fail with 404 even
    -- if the preflight request /supported/ the usage of the HTTP method with CORS.
    --
    -- The implementation does not distinguish between simple requests and requests
    -- that require preflight. The client is free to omit a preflight request or do
    -- a preflight request in cases when it wouldn't be required.
    --
    -- For application authors it is strongly recommended to take into account the
    -- security considerations in section 6.3 of <http://www.w3.org/TR/cors>.
    --
    -- /TODO/
    --
    -- * We may consider adding optional enforcment aspects to this module: we may
    --   check if a request respects our origin restrictions and we may check that a
    --   CORS request respects the restrictions that we publish in the preflight
    --   responses.
    --
    -- * Even though slightly out of scope we may (optionally) check if
    --   host header matches the actual host of the resource, since clients
    --   using CORS may expect this, since this check is recommended in
    --   <http://www.w3.org/TR/cors>.
    --
    -- * We may consider integrating CORS policy handling more closely with the
    --   handling of the source, for instance by integrating with 'ActionM' from
    --   scotty.
    --
    cors
        ∷ (WAI.Request → Maybe CorsResourcePolicy) -- ^ A value of 'Nothing' indicates that the resource is not available for CORS
        → WAI.Middleware
    cors :: (Request -> Maybe CorsResourcePolicy) -> Middleware
    cors policyPattern :: Request -> Maybe CorsResourcePolicy
    policyPattern app :: Application
    app r :: Request
    r respond :: Response -> IO ResponseReceived
    respond
        -- We don't handle websockets, even if they include an @Origin@ header
        | Request -> Bool
    isWebSocketsReq Request
    r = IO ResponseReceived
    runApp

        | Just policy :: CorsResourcePolicy
    policy ← Request -> Maybe CorsResourcePolicy
    policyPattern Request
    r = case Maybe Origin
    hdrOrigin of

            -- No origin header: reject request
            Nothing → if CorsResourcePolicy -> Bool
    corsRequireOrigin CorsResourcePolicy
    policy
                then Response -> IO ResponseReceived
    res (Response -> IO ResponseReceived)
    -> Response -> IO ResponseReceived
    forall a b. (a -> b) -> a -> b
    $ Origin -> Response
    corsFailure "Origin header is missing"
                else IO ResponseReceived
    runApp

            -- Origin header: apply CORS policy to request
            Just origin :: Origin
    origin → CorsResourcePolicy -> Origin -> IO ResponseReceived
    applyCorsPolicy CorsResourcePolicy
    policy Origin
    origin

        | Bool
    otherwise = IO ResponseReceived
    runApp
      where
        res :: Response -> IO ResponseReceived
    res = Response -> IO ResponseReceived
    respond
        runApp :: IO ResponseReceived
    runApp = Application
    app Request
    r Response -> IO ResponseReceived
    respond

        -- Lookup the HTTP origin request header
        --
        hdrOrigin :: Maybe Origin
    hdrOrigin = HeaderName -> [(HeaderName, Origin)] -> Maybe Origin
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup "origin" (Request -> [(HeaderName, Origin)]
    WAI.requestHeaders Request
    r)

        -- Process a CORS request
        --
        applyCorsPolicy :: CorsResourcePolicy -> Origin -> IO ResponseReceived
    applyCorsPolicy policy :: CorsResourcePolicy
    policy origin :: Origin
    origin = do

            -- The error continuation
            let err :: String -> IO ResponseReceived
    err e :: String
    e = if CorsResourcePolicy -> Bool
    corsIgnoreFailures CorsResourcePolicy
    policy
                then IO ResponseReceived
    runApp
                else Response -> IO ResponseReceived
    res (Response -> IO ResponseReceived)
    -> Response -> IO ResponseReceived
    forall a b. (a -> b) -> a -> b
    $ Origin -> Response
    corsFailure (String -> Origin
    B8.pack String
    e)

            -- Match request origin with corsOrigins from policy
            let respOriginOrErr :: Either String (Maybe (Origin, Bool))
    respOriginOrErr = case CorsResourcePolicy -> Maybe ([Origin], Bool)
    corsOrigins CorsResourcePolicy
    policy of
                    Nothing → Maybe (Origin, Bool) -> Either String (Maybe (Origin, Bool))
    forall (m :: * -> *) a. Monad m => a -> m a
    return Maybe (Origin, Bool)
    forall a. Maybe a
    Nothing
                    Just (originList :: [Origin]
    originList, withCreds :: Bool
    withCreds) → if Origin
    origin Origin -> [Origin] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [Origin]
    originList
                        then Maybe (Origin, Bool) -> Either String (Maybe (Origin, Bool))
    forall a b. b -> Either a b
    Right (Maybe (Origin, Bool) -> Either String (Maybe (Origin, Bool)))
    -> Maybe (Origin, Bool) -> Either String (Maybe (Origin, Bool))
    forall a b. (a -> b) -> a -> b
    $ (Origin, Bool) -> Maybe (Origin, Bool)
    forall a. a -> Maybe a
    Just (Origin
    origin, Bool
    withCreds)
                        else String -> Either String (Maybe (Origin, Bool))
    forall a b. a -> Either a b
    Left (String -> Either String (Maybe (Origin, Bool)))
    -> String -> Either String (Maybe (Origin, Bool))
    forall a b. (a -> b) -> a -> b
    $ "Unsupported origin: " String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ Origin -> String
    B8.unpack Origin
    origin

            case Either String (Maybe (Origin, Bool))
    respOriginOrErr of
                Left e :: String
    e → String -> IO ResponseReceived
    err String
    e
                Right respOrigin :: Maybe (Origin, Bool)
    respOrigin → do

                    -- Determine headers that are common to actuall responses and preflight responses
                    let ch :: [(HeaderName, Origin)]
    ch = Maybe (Origin, Bool) -> Bool -> [(HeaderName, Origin)]
    commonCorsHeaders Maybe (Origin, Bool)
    respOrigin (CorsResourcePolicy -> Bool
    corsVaryOrigin CorsResourcePolicy
    policy)

                    case Request -> Origin
    WAI.requestMethod Request
    r of

                        -- Preflight CORS request
                        "OPTIONS" → ExceptT String IO [(HeaderName, Origin)]
    -> IO (Either String [(HeaderName, Origin)])
    forall e (m :: * -> *) a. ExceptT e m a -> m (Either e a)
    runExceptT (CorsResourcePolicy -> ExceptT String IO [(HeaderName, Origin)]
    forall (μ :: * -> *).
    (Functor μ, Monad μ) =>
    CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    preflightHeaders CorsResourcePolicy
    policy) IO (Either String [(HeaderName, Origin)])
    -> (Either String [(HeaderName, Origin)] -> IO ResponseReceived)
    -> IO ResponseReceived
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= \case
                            Left e :: String
    e → String -> IO ResponseReceived
    err String
    e
                            Right headers :: [(HeaderName, Origin)]
    headers → Response -> IO ResponseReceived
    res (Response -> IO ResponseReceived)
    -> Response -> IO ResponseReceived
    forall a b. (a -> b) -> a -> b
    $ Status -> [(HeaderName, Origin)] -> ByteString -> Response
    WAI.responseLBS Status
    HTTP.ok200 ([(HeaderName, Origin)]
    ch [(HeaderName, Origin)]
    -> [(HeaderName, Origin)] -> [(HeaderName, Origin)]
    forall α. Monoid α => α -> α -> α
    ⊕ [(HeaderName, Origin)]
    headers) ""

                        -- Actual CORS request
                        _ → [(HeaderName, Origin)] -> Middleware
    addHeaders ([(HeaderName, Origin)]
    ch [(HeaderName, Origin)]
    -> [(HeaderName, Origin)] -> [(HeaderName, Origin)]
    forall α. Monoid α => α -> α -> α
    ⊕ CorsResourcePolicy -> [(HeaderName, Origin)]
    respCorsHeaders CorsResourcePolicy
    policy) Application
    app Request
    r Response -> IO ResponseReceived
    respond

        -- Compute HTTP response headers for a preflight request
        --
        preflightHeaders ∷ (Functor μ, Monad μ) ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
        preflightHeaders :: CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    preflightHeaders policy :: CorsResourcePolicy
    policy = [[(HeaderName, Origin)]] -> [(HeaderName, Origin)]
    forall (t :: * -> *) a. Foldable t => t [a] -> [a]
    concat ([[(HeaderName, Origin)]] -> [(HeaderName, Origin)])
    -> ExceptT String μ [[(HeaderName, Origin)]]
    -> ExceptT String μ [(HeaderName, Origin)]
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> [ExceptT String μ [(HeaderName, Origin)]]
    -> ExceptT String μ [[(HeaderName, Origin)]]
    forall (t :: * -> *) (m :: * -> *) a.
    (Traversable t, Monad m) =>
    t (m a) -> m (t a)
    sequence
            [ CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    forall (μ :: * -> *).
    Monad μ =>
    CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    hdrReqMethod CorsResourcePolicy
    policy
            , CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    forall (μ :: * -> *).
    Monad μ =>
    CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    hdrRequestHeader CorsResourcePolicy
    policy
            , CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    forall (μ :: * -> *).
    Monad μ =>
    CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    hdrMaxAge CorsResourcePolicy
    policy
            ]

        hdrMaxAge ∷ Monad μ ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
        hdrMaxAge :: CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    hdrMaxAge policy :: CorsResourcePolicy
    policy = case CorsResourcePolicy -> Maybe Int
    corsMaxAge CorsResourcePolicy
    policy of
            Nothing → [(HeaderName, Origin)] -> ExceptT String μ [(HeaderName, Origin)]
    forall (m :: * -> *) a. Monad m => a -> m a
    return []
            Just secs :: Int
    secs → [(HeaderName, Origin)] -> ExceptT String μ [(HeaderName, Origin)]
    forall (m :: * -> *) a. Monad m => a -> m a
    return [("Access-Control-Max-Age", Int -> Origin
    forall α β. (IsString α, Show β) => β -> α
    sshow Int
    secs)]

        hdrReqMethod ∷ Monad μ ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
        hdrReqMethod :: CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    hdrReqMethod policy :: CorsResourcePolicy
    policy = case HeaderName -> [(HeaderName, Origin)] -> Maybe Origin
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup "Access-Control-Request-Method" (Request -> [(HeaderName, Origin)]
    WAI.requestHeaders Request
    r) of
            Nothing → String -> ExceptT String μ [(HeaderName, Origin)]
    forall e (m :: * -> *) a. MonadError e m => e -> m a
    throwError "Access-Control-Request-Method header is missing in CORS preflight request"
            Just x :: Origin
    x → if  Origin
    x Origin -> [Origin] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [Origin]
    supportedMethods
                then [(HeaderName, Origin)] -> ExceptT String μ [(HeaderName, Origin)]
    forall (m :: * -> *) a. Monad m => a -> m a
    return [("Access-Control-Allow-Methods", [Origin] -> Origin
    hdrL [Origin]
    supportedMethods)]
                else String -> ExceptT String μ [(HeaderName, Origin)]
    forall e (m :: * -> *) a. MonadError e m => e -> m a
    throwError
                    (String -> ExceptT String μ [(HeaderName, Origin)])
    -> String -> ExceptT String μ [(HeaderName, Origin)]
    forall a b. (a -> b) -> a -> b
    $ "Method requested in Access-Control-Request-Method of CORS request is not supported; requested: "
                    String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ Origin -> String
    B8.unpack Origin
    x
                    String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ "; supported are "
                    String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ Origin -> String
    B8.unpack ([Origin] -> Origin
    hdrL [Origin]
    supportedMethods)
                    String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ "."
          where
             supportedMethods :: [Origin]
    supportedMethods = CorsResourcePolicy -> [Origin]
    corsMethods CorsResourcePolicy
    policy [Origin] -> [Origin] -> [Origin]
    forall a. Eq a => [a] -> [a] -> [a]
    `union` [Origin]
    simpleMethods

        hdrRequestHeader ∷ Monad μ ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
        hdrRequestHeader :: CorsResourcePolicy -> ExceptT String μ [(HeaderName, Origin)]
    hdrRequestHeader policy :: CorsResourcePolicy
    policy = case HeaderName -> [(HeaderName, Origin)] -> Maybe Origin
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup "Access-Control-Request-Headers" (Request -> [(HeaderName, Origin)]
    WAI.requestHeaders Request
    r) of
            Nothing → [(HeaderName, Origin)] -> ExceptT String μ [(HeaderName, Origin)]
    forall (m :: * -> *) a. Monad m => a -> m a
    return []
            Just hdrsBytes :: Origin
    hdrsBytes → do
                [HeaderName]
    hdrs ← (String -> ExceptT String μ [HeaderName])
    -> ([HeaderName] -> ExceptT String μ [HeaderName])
    -> Either String [HeaderName]
    -> ExceptT String μ [HeaderName]
    forall a c b. (a -> c) -> (b -> c) -> Either a b -> c
    either String -> ExceptT String μ [HeaderName]
    forall e (m :: * -> *) a. MonadError e m => e -> m a
    throwError [HeaderName] -> ExceptT String μ [HeaderName]
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Either String [HeaderName] -> ExceptT String μ [HeaderName])
    -> Either String [HeaderName] -> ExceptT String μ [HeaderName]
    forall a b. (a -> b) -> a -> b
    $ Parser [HeaderName] -> Origin -> Either String [HeaderName]
    forall a. Parser a -> Origin -> Either String a
    P.parseOnly Parser [HeaderName]
    httpHeaderNameListParser Origin
    hdrsBytes
                if [HeaderName]
    hdrs [HeaderName] -> [HeaderName] -> Bool
    forall α. Eq α => [α] -> [α] -> Bool
    `isSubsetOf` [HeaderName]
    supportedHeaders
                    then [(HeaderName, Origin)] -> ExceptT String μ [(HeaderName, Origin)]
    forall (m :: * -> *) a. Monad m => a -> m a
    return [("Access-Control-Allow-Headers", [HeaderName] -> Origin
    hdrLI [HeaderName]
    supportedHeaders)]
                    else String -> ExceptT String μ [(HeaderName, Origin)]
    forall e (m :: * -> *) a. MonadError e m => e -> m a
    throwError
                        (String -> ExceptT String μ [(HeaderName, Origin)])
    -> String -> ExceptT String μ [(HeaderName, Origin)]
    forall a b. (a -> b) -> a -> b
    $ "HTTP header requested in Access-Control-Request-Headers of CORS request is not supported; requested: "
                        String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ Origin -> String
    B8.unpack ([HeaderName] -> Origin
    hdrLI [HeaderName]
    hdrs)
                        String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ "; supported are "
                        String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ Origin -> String
    B8.unpack ([HeaderName] -> Origin
    hdrLI [HeaderName]
    supportedHeaders)
                        String -> ShowS
    forall α. Monoid α => α -> α -> α
    ⊕ "."
          where
            supportedHeaders :: [HeaderName]
    supportedHeaders = CorsResourcePolicy -> [HeaderName]
    corsRequestHeaders CorsResourcePolicy
    policy [HeaderName] -> [HeaderName] -> [HeaderName]
    forall a. Eq a => [a] -> [a] -> [a]
    `union` [HeaderName]
    simpleHeadersWithoutContentType

        simpleHeadersWithoutContentType :: [HeaderName]
    simpleHeadersWithoutContentType = [HeaderName]
    simpleHeaders [HeaderName] -> [HeaderName] -> [HeaderName]
    forall a. Eq a => [a] -> [a] -> [a]
    \\ ["content-type"]

        -- HTTP response headers that are common to normal and preflight CORS responses
        --
        commonCorsHeaders ∷ Maybe (Origin, Bool) → Bool → HTTP.ResponseHeaders
        commonCorsHeaders :: Maybe (Origin, Bool) -> Bool -> [(HeaderName, Origin)]
    commonCorsHeaders Nothing _ = [("Access-Control-Allow-Origin", "*")]
        commonCorsHeaders (Just (o :: Origin
    o, creds :: Bool
    creds)) vary :: Bool
    vary = []
            [(HeaderName, Origin)]
    -> [(HeaderName, Origin)] -> [(HeaderName, Origin)]
    forall α. Monoid α => α -> α -> α
    ⊕ (Bool
    True Bool -> (HeaderName, Origin) -> [(HeaderName, Origin)]
    forall (f :: * -> *) a.
    (Applicative f, Monoid (f a)) =>
    Bool -> a -> f a
    ?? ("Access-Control-Allow-Origin", Origin
    o))
            [(HeaderName, Origin)]
    -> [(HeaderName, Origin)] -> [(HeaderName, Origin)]
    forall α. Monoid α => α -> α -> α
    ⊕ (Bool
    creds Bool -> (HeaderName, Origin) -> [(HeaderName, Origin)]
    forall (f :: * -> *) a.
    (Applicative f, Monoid (f a)) =>
    Bool -> a -> f a
    ?? ("Access-Control-Allow-Credentials", "true"))
            [(HeaderName, Origin)]
    -> [(HeaderName, Origin)] -> [(HeaderName, Origin)]
    forall α. Monoid α => α -> α -> α
    ⊕ (Bool
    vary Bool -> (HeaderName, Origin) -> [(HeaderName, Origin)]
    forall (f :: * -> *) a.
    (Applicative f, Monoid (f a)) =>
    Bool -> a -> f a
    ?? ("Vary", "Origin"))
          where
            ?? :: Bool -> a -> f a
    (??) a :: Bool
    a b :: a
    b = if Bool
    a then a -> f a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure a
    b else f a
    forall a. Monoid a => a
    mempty

        -- HTTP response headers that are only used with normal CORS responses
        --
        respCorsHeaders ∷ CorsResourcePolicy → HTTP.ResponseHeaders
        respCorsHeaders :: CorsResourcePolicy -> [(HeaderName, Origin)]
    respCorsHeaders policy :: CorsResourcePolicy
    policy = [Maybe (HeaderName, Origin)] -> [(HeaderName, Origin)]
    forall a. [Maybe a] -> [a]
    catMaybes
            [ ([HeaderName] -> (HeaderName, Origin))
    -> Maybe [HeaderName] -> Maybe (HeaderName, Origin)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap (\x :: [HeaderName]
    x → ("Access-Control-Expose-Headers", [HeaderName] -> Origin
    hdrLI [HeaderName]
    x)) (CorsResourcePolicy -> Maybe [HeaderName]
    corsExposedHeaders CorsResourcePolicy
    policy)
            ]

    -- | A CORS middleware that supports simple cross-origin requests for all
    -- resources.
    --
    -- This middleware does not check if the resource corresponds to the
    -- restrictions for simple requests. This is in accordance with
    -- <http://www.w3.org/TR/cors/>. It is the responsibility of the
    -- client (user-agent) to enforce CORS policy. The role of the server
    -- is to provide the client with the respective policy constraints.
    --
    -- It is out of the scope of the this module if the server chooses to
    -- enforce rules on its resources in relation to CORS policy itself.
    --
    simpleCors ∷ WAI.Middleware
    simpleCors :: Middleware
    simpleCors = (Request -> Maybe CorsResourcePolicy) -> Middleware
    cors (Maybe CorsResourcePolicy -> Request -> Maybe CorsResourcePolicy
    forall a b. a -> b -> a
    const (Maybe CorsResourcePolicy -> Request -> Maybe CorsResourcePolicy)
    -> Maybe CorsResourcePolicy -> Request -> Maybe CorsResourcePolicy
    forall a b. (a -> b) -> a -> b
    $ CorsResourcePolicy -> Maybe CorsResourcePolicy
    forall a. a -> Maybe a
    Just CorsResourcePolicy
    simpleCorsResourcePolicy)

    -- -------------------------------------------------------------------------- --
    -- Definition from Standards

    -- | Simple HTTP response headers as defined in <https://www.w3.org/TR/cors/#simple-response-header>
    --
    simpleResponseHeaders ∷ [HTTP.HeaderName]
    simpleResponseHeaders :: [HeaderName]
    simpleResponseHeaders =
        [ "Cache-Control"
        , "Content-Language"
        , "Content-Type"
        , "Expires"
        , "Last-Modified"
        , "Pragma"
        ]

    -- | Simple HTTP headers are defined in <https://www.w3.org/TR/cors/#simple-header>
    simpleHeaders ∷ [HTTP.HeaderName]
    simpleHeaders :: [HeaderName]
    simpleHeaders =
        [ "Accept"
        , "Accept-Language"
        , "Content-Language"
        , "Content-Type"
        ]

    -- | Simple content types are defined in <https://www.w3.org/TR/cors/#simple-header>
    simpleContentTypes ∷ [CI.CI B8.ByteString]
    simpleContentTypes :: [HeaderName]
    simpleContentTypes =
        [ "application/x-www-form-urlencoded"
        , "multipart/form-data"
        , "text/plain"
        ]

    -- | Simple HTTP methods as defined in <https://www.w3.org/TR/cors/#simple-method>
    --
    simpleMethods ∷ [HTTP.Method]
    simpleMethods :: [Origin]
    simpleMethods =
        [ "GET"
        , "HEAD"
        , "POST"
        ]

    -- | Whether the given method and headers constitute a simple request,
    -- i.e. the method is simple, all headers are simple, and, if a POST request,
    -- the content-type is simple.
    isSimple ∷ HTTP.Method → HTTP.RequestHeaders → Bool
    isSimple :: Origin -> [(HeaderName, Origin)] -> Bool
    isSimple method :: Origin
    method headers :: [(HeaderName, Origin)]
    headers
        = Origin
    method Origin -> [Origin] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [Origin]
    simpleMethods
        Bool -> Bool -> Bool
    ∧ ((HeaderName, Origin) -> HeaderName)
    -> [(HeaderName, Origin)] -> [HeaderName]
    forall a b. (a -> b) -> [a] -> [b]
    map (HeaderName, Origin) -> HeaderName
    forall a b. (a, b) -> a
    fst [(HeaderName, Origin)]
    headers [HeaderName] -> [HeaderName] -> Bool
    forall α. Eq α => [α] -> [α] -> Bool
    `isSubsetOf` [HeaderName]
    simpleHeaders
        Bool -> Bool -> Bool
    ∧ case (Origin
    method, HeaderName -> [(HeaderName, Origin)] -> Maybe Origin
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup "content-type" [(HeaderName, Origin)]
    headers) of
            ("POST", Just x :: Origin
    x) → Origin -> HeaderName
    forall s. FoldCase s => s -> CI s
    CI.mk Origin
    x HeaderName -> [HeaderName] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [HeaderName]
    simpleContentTypes
            _ → Bool
    True

    -- | Valid characters for HTTP header names according to RFC2616 (section 4.2)
    --
    isHttpHeaderNameChar ∷ Char → Bool
    isHttpHeaderNameChar :: Char -> Bool
    isHttpHeaderNameChar c :: Char
    c = (Char
    c Char -> Char -> Bool
    forall α. Ord α => α -> α -> Bool
    ≥ Int -> Char
    forall a. Enum a => Int -> a
    toEnum 33) Bool -> Bool -> Bool
    && (Char
    c Char -> Char -> Bool
    forall α. Ord α => α -> α -> Bool
    ≤ Int -> Char
    forall a. Enum a => Int -> a
    toEnum 126) Bool -> Bool -> Bool
    && String -> Char -> Bool
    P.notInClass "()<>@,;:\\\"/[]?={}" Char
    c

    httpHeaderNameParser ∷ P.Parser HTTP.HeaderName
    httpHeaderNameParser :: Parser HeaderName
    httpHeaderNameParser = String -> HeaderName
    forall a. IsString a => String -> a
    fromString (String -> HeaderName) -> Parser Origin String -> Parser HeaderName
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Parser Origin Char -> Parser Origin String
    forall (f :: * -> *) a. Alternative f => f a -> f [a]
    P.many1 ((Char -> Bool) -> Parser Origin Char
    P.satisfy Char -> Bool
    isHttpHeaderNameChar) Parser HeaderName -> String -> Parser HeaderName
    forall i a. Parser i a -> String -> Parser i a
    P.<?> "HTTP Header Name"

    -- -------------------------------------------------------------------------- --
    -- Generic Tools

    -- | A comma separated list of whitespace surounded HTTP header names.
    --
    -- Note that 'P.space' includes @SP@ (32), @HT@ (9), @LF@ (10), @VT@ (11),
    -- @NP@ (12), and @CR@ (13). RFC 2616 (2.2) only defines @SP@ (32) and
    -- @LWS = [CRLF] 1*(SP | HT)@ as whitespace. That's fine here since neither
    -- of these characters is allowed in header names.
    --
    httpHeaderNameListParser ∷ P.Parser [HTTP.HeaderName]
    httpHeaderNameListParser :: Parser [HeaderName]
    httpHeaderNameListParser = Parser Origin String
    spaces Parser Origin String -> Parser [HeaderName] -> Parser [HeaderName]
    forall (f :: * -> *) a b. Applicative f => f a -> f b -> f b
    *> Parser HeaderName -> Parser Origin Char -> Parser [HeaderName]
    forall (f :: * -> *) a s. Alternative f => f a -> f s -> f [a]
    P.sepBy (Parser HeaderName
    httpHeaderNameParser Parser HeaderName -> Parser Origin String -> Parser HeaderName
    forall (f :: * -> *) a b. Applicative f => f a -> f b -> f a
    <* Parser Origin String
    spaces) (Char -> Parser Origin Char
    P.char ',') Parser [HeaderName] -> Parser Origin String -> Parser [HeaderName]
    forall (f :: * -> *) a b. Applicative f => f a -> f b -> f a
    <* Parser Origin String
    spaces
      where
        spaces :: Parser Origin String
    spaces = Parser Origin Char -> Parser Origin String
    forall (m :: * -> *) a. MonadPlus m => m a -> m [a]
    P.many' Parser Origin Char
    P.space

    sshow ∷ (IsString α, Show β) ⇒ β → α
    sshow :: β -> α
    sshow = String -> α
    forall a. IsString a => String -> a
    fromString (String -> α) -> (β -> String) -> β -> α
    forall β γ α. (β -> γ) -> (α -> β) -> α -> γ
    ∘ β -> String
    forall a. Show a => a -> String
    show

    isSubsetOf ∷ Eq α ⇒ [α] → [α] → Bool
    isSubsetOf :: [α] -> [α] -> Bool
    isSubsetOf l1 :: [α]
    l1 l2 :: [α]
    l2 = [α] -> [α] -> [α]
    forall a. Eq a => [a] -> [a] -> [a]
    intersect [α]
    l1 [α]
    l2 [α] -> [α] -> Bool
    forall α. Eq α => α -> α -> Bool
    ≡ [α]
    l1

    -- | Add HTTP headers to a WAI response
    --
    addHeaders ∷ HTTP.ResponseHeaders → WAI.Middleware
    addHeaders :: [(HeaderName, Origin)] -> Middleware
    addHeaders hdrs :: [(HeaderName, Origin)]
    hdrs app :: Application
    app req :: Request
    req respond :: Response -> IO ResponseReceived
    respond = Application
    app Request
    req ((Response -> IO ResponseReceived) -> IO ResponseReceived)
    -> (Response -> IO ResponseReceived) -> IO ResponseReceived
    forall a b. (a -> b) -> a -> b
    $ \response :: Response
    response → do
        let (st :: Status
    st, headers :: [(HeaderName, Origin)]
    headers, streamHandle :: (StreamingBody -> IO ResponseReceived) -> IO ResponseReceived
    streamHandle) = Response
    -> (Status, [(HeaderName, Origin)],
        (StreamingBody -> IO ResponseReceived) -> IO ResponseReceived)
    forall a.
    Response
    -> (Status, [(HeaderName, Origin)],
        (StreamingBody -> IO a) -> IO a)
    WAI.responseToStream Response
    response
        (StreamingBody -> IO ResponseReceived) -> IO ResponseReceived
    streamHandle ((StreamingBody -> IO ResponseReceived) -> IO ResponseReceived)
    -> (StreamingBody -> IO ResponseReceived) -> IO ResponseReceived
    forall a b. (a -> b) -> a -> b
    $ \streamBody :: StreamingBody
    streamBody →
            Response -> IO ResponseReceived
    respond (Response -> IO ResponseReceived)
    -> Response -> IO ResponseReceived
    forall a b. (a -> b) -> a -> b
    $ Status -> [(HeaderName, Origin)] -> StreamingBody -> Response
    WAI.responseStream Status
    st ([(HeaderName, Origin)]
    headers [(HeaderName, Origin)]
    -> [(HeaderName, Origin)] -> [(HeaderName, Origin)]
    forall α. Monoid α => α -> α -> α
    ⊕ [(HeaderName, Origin)]
    hdrs) StreamingBody
    streamBody

    -- | Format a list of 'HTTP.HeaderName's such that it can be used as
    -- an HTTP header value
    --
    hdrLI ∷ [HTTP.HeaderName] → B8.ByteString
    hdrLI :: [HeaderName] -> Origin
    hdrLI l :: [HeaderName]
    l = Origin -> [Origin] -> Origin
    B8.intercalate ", " ((HeaderName -> Origin) -> [HeaderName] -> [Origin]
    forall a b. (a -> b) -> [a] -> [b]
    map HeaderName -> Origin
    forall s. CI s -> s
    CI.original [HeaderName]
    l)

    -- | Format a list of 'B8.ByteString's such that it can be used as
    -- an HTTP header value
    --
    hdrL ∷ [B8.ByteString] → B8.ByteString
    hdrL :: [Origin] -> Origin
    hdrL = Origin -> [Origin] -> Origin
    B8.intercalate ", "

    corsFailure
        ∷ B8.ByteString -- ^ body
        → WAI.Response
    corsFailure :: Origin -> Response
    corsFailure msg :: Origin
    msg = Status -> [(HeaderName, Origin)] -> ByteString -> Response
    WAI.responseLBS Status
    HTTP.status400 [("Content-Type", "text/html; charset-utf-8")] (Origin -> ByteString
    LB8.fromStrict Origin
    msg)

    -- Copied from the [wai-websocket package](https://github.com/yesodweb/wai/blob/master/wai-websockets/Network/Wai/Handler/WebSockets.hs#L21)
    --
    isWebSocketsReq
        ∷ WAI.Request
        → Bool
    isWebSocketsReq :: Request -> Bool
    isWebSocketsReq req :: Request
    req =
        (Origin -> HeaderName) -> Maybe Origin -> Maybe HeaderName
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap Origin -> HeaderName
    forall s. FoldCase s => s -> CI s
    CI.mk (HeaderName -> [(HeaderName, Origin)] -> Maybe Origin
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup "upgrade" ([(HeaderName, Origin)] -> Maybe Origin)
    -> [(HeaderName, Origin)] -> Maybe Origin
    forall a b. (a -> b) -> a -> b
    $ Request -> [(HeaderName, Origin)]
    WAI.requestHeaders Request
    req) Maybe HeaderName -> Maybe HeaderName -> Bool
    forall α. Eq α => α -> α -> Bool
    == HeaderName -> Maybe HeaderName
    forall a. a -> Maybe a
    Just "websocket"