    {-# 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 ([ByteString], Bool)
    corsOrigins ∷ !(Maybe ([Origin], Bool))

        -- | HTTP methods that are allowed in CORS requests.
        --
        , CorsResourcePolicy -> [ByteString]
    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 -> [CI ByteString]
    corsRequestHeaders ∷ ![HTTP.HeaderName]

        -- | Field names of HTTP headers that are exposed to the client in the response.
        --
        , CorsResourcePolicy -> Maybe [CI ByteString]
    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 -> [Char]
    (Int -> CorsResourcePolicy -> ShowS)
    -> (CorsResourcePolicy -> [Char])
    -> ([CorsResourcePolicy] -> ShowS)
    -> Show CorsResourcePolicy
    forall a.
    (Int -> a -> ShowS) -> (a -> [Char]) -> ([a] -> ShowS) -> Show a
    $cshowsPrec :: Int -> CorsResourcePolicy -> ShowS
    showsPrec :: Int -> CorsResourcePolicy -> ShowS
    $cshow :: CorsResourcePolicy -> [Char]
    show :: CorsResourcePolicy -> [Char]
    $cshowList :: [CorsResourcePolicy] -> ShowS
    showList :: [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
    $creadsPrec :: Int -> ReadS CorsResourcePolicy
    readsPrec :: Int -> ReadS CorsResourcePolicy
    $creadList :: ReadS [CorsResourcePolicy]
    readList :: ReadS [CorsResourcePolicy]
    $creadPrec :: ReadPrec CorsResourcePolicy
    readPrec :: ReadPrec CorsResourcePolicy
    $creadListPrec :: ReadPrec [CorsResourcePolicy]
    readListPrec :: ReadPrec [CorsResourcePolicy]
    Read,CorsResourcePolicy -> CorsResourcePolicy -> Bool
    (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> (CorsResourcePolicy -> CorsResourcePolicy -> Bool)
    -> Eq CorsResourcePolicy
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    $c== :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    == :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $c/= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    /= :: 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
    $ccompare :: CorsResourcePolicy -> CorsResourcePolicy -> Ordering
    compare :: CorsResourcePolicy -> CorsResourcePolicy -> Ordering
    $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
    >= :: CorsResourcePolicy -> CorsResourcePolicy -> Bool
    $cmax :: CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    max :: CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    $cmin :: CorsResourcePolicy -> CorsResourcePolicy -> CorsResourcePolicy
    min :: CorsResourcePolicy -> CorsResourcePolicy -> 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 = CorsResourcePolicy
        { corsOrigins :: Maybe ([ByteString], Bool)
    corsOrigins = Maybe ([ByteString], Bool)
    forall a. Maybe a
    Nothing
        , corsMethods :: [ByteString]
    corsMethods = [ByteString]
    simpleMethods
        , corsRequestHeaders :: [CI ByteString]
    corsRequestHeaders = []
        , corsExposedHeaders :: Maybe [CI ByteString]
    corsExposedHeaders = Maybe [CI ByteString]
    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 Request -> Maybe CorsResourcePolicy
    policyPattern Application
    app Request
    r 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 CorsResourcePolicy
    policy ← Request -> Maybe CorsResourcePolicy
    policyPattern Request
    r = case Maybe ByteString
    hdrOrigin of

            -- No origin header: reject request
            Maybe ByteString
    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
    $ ByteString -> Response
    corsFailure ByteString
    "Origin header is missing"
                else IO ResponseReceived
    runApp

            -- Origin header: apply CORS policy to request
            Just ByteString
    origin → CorsResourcePolicy -> ByteString -> IO ResponseReceived
    applyCorsPolicy CorsResourcePolicy
    policy ByteString
    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 ByteString
    hdrOrigin = CI ByteString -> ResponseHeaders -> Maybe ByteString
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup CI ByteString
    "origin" (Request -> ResponseHeaders
    WAI.requestHeaders Request
    r)

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

            -- The error continuation
            let err :: [Char] -> IO ResponseReceived
    err [Char]
    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
    $ ByteString -> Response
    corsFailure ([Char] -> ByteString
    B8.pack [Char]
    e)

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

            case Either [Char] (Maybe (ByteString, Bool))
    respOriginOrErr of
                Left [Char]
    e → [Char] -> IO ResponseReceived
    err [Char]
    e
                Right Maybe (ByteString, Bool)
    respOrigin → do

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

                    case Request -> ByteString
    WAI.requestMethod Request
    r of

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

                        -- Actual CORS request
                        ByteString
    _ → ResponseHeaders -> Middleware
    addHeaders (ResponseHeaders
    ch ResponseHeaders -> ResponseHeaders -> ResponseHeaders
    forall α. Monoid α => α -> α -> α
    ⊕ CorsResourcePolicy -> ResponseHeaders
    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 :: forall (μ :: * -> *).
    (Functor μ, Monad μ) =>
    CorsResourcePolicy -> ExceptT [Char] μ ResponseHeaders
    preflightHeaders CorsResourcePolicy
    policy = [ResponseHeaders] -> ResponseHeaders
    forall (t :: * -> *) a. Foldable t => t [a] -> [a]
    concat ([ResponseHeaders] -> ResponseHeaders)
    -> ExceptT [Char] μ [ResponseHeaders]
    -> ExceptT [Char] μ ResponseHeaders
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> [ExceptT [Char] μ ResponseHeaders]
    -> ExceptT [Char] μ [ResponseHeaders]
    forall (t :: * -> *) (m :: * -> *) a.
    (Traversable t, Monad m) =>
    t (m a) -> m (t a)
    forall (m :: * -> *) a. Monad m => [m a] -> m [a]
    sequence
            [ CorsResourcePolicy -> ExceptT [Char] μ ResponseHeaders
    forall (μ :: * -> *).
    Monad μ =>
    CorsResourcePolicy -> ExceptT [Char] μ ResponseHeaders
    hdrReqMethod CorsResourcePolicy
    policy
            , CorsResourcePolicy -> ExceptT [Char] μ ResponseHeaders
    forall (μ :: * -> *).
    Monad μ =>
    CorsResourcePolicy -> ExceptT [Char] μ ResponseHeaders
    hdrRequestHeader CorsResourcePolicy
    policy
            , CorsResourcePolicy -> ExceptT [Char] μ ResponseHeaders
    forall (μ :: * -> *).
    Monad μ =>
    CorsResourcePolicy -> ExceptT [Char] μ ResponseHeaders
    hdrMaxAge CorsResourcePolicy
    policy
            ]

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

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

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

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

        -- HTTP response headers that are common to normal and preflight CORS responses
        --
        commonCorsHeaders ∷ Maybe (Origin, Bool) → Bool → HTTP.ResponseHeaders
        commonCorsHeaders :: Maybe (ByteString, Bool) -> Bool -> ResponseHeaders
    commonCorsHeaders Maybe (ByteString, Bool)
    Nothing Bool
    _ = [(CI ByteString
    "Access-Control-Allow-Origin", ByteString
    "*")]
        commonCorsHeaders (Just (ByteString
    o, Bool
    creds)) Bool
    vary = []
            ResponseHeaders -> ResponseHeaders -> ResponseHeaders
    forall α. Monoid α => α -> α -> α
    ⊕ (Bool
    True Bool -> Header -> ResponseHeaders
    forall {f :: * -> *} {a}.
    (Applicative f, Monoid (f a)) =>
    Bool -> a -> f a
    ?? (CI ByteString
    "Access-Control-Allow-Origin", ByteString
    o))
            ResponseHeaders -> ResponseHeaders -> ResponseHeaders
    forall α. Monoid α => α -> α -> α
    ⊕ (Bool
    creds Bool -> Header -> ResponseHeaders
    forall {f :: * -> *} {a}.
    (Applicative f, Monoid (f a)) =>
    Bool -> a -> f a
    ?? (CI ByteString
    "Access-Control-Allow-Credentials", ByteString
    "true"))
            ResponseHeaders -> ResponseHeaders -> ResponseHeaders
    forall α. Monoid α => α -> α -> α
    ⊕ (Bool
    vary Bool -> Header -> ResponseHeaders
    forall {f :: * -> *} {a}.
    (Applicative f, Monoid (f a)) =>
    Bool -> a -> f a
    ?? (CI ByteString
    "Vary", ByteString
    "Origin"))
          where
            ?? :: Bool -> a -> f a
    (??) Bool
    a a
    b = if Bool
    a then a -> f a
    forall a. 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 -> ResponseHeaders
    respCorsHeaders CorsResourcePolicy
    policy = [Maybe Header] -> ResponseHeaders
    forall a. [Maybe a] -> [a]
    catMaybes
            [ ([CI ByteString] -> Header)
    -> Maybe [CI ByteString] -> Maybe Header
    forall a b. (a -> b) -> Maybe a -> Maybe b
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap (\[CI ByteString]
    x → (CI ByteString
    "Access-Control-Expose-Headers", [CI ByteString] -> ByteString
    hdrLI [CI ByteString]
    x)) (CorsResourcePolicy -> Maybe [CI ByteString]
    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 :: [CI ByteString]
    simpleResponseHeaders =
        [ CI ByteString
    "Cache-Control"
        , CI ByteString
    "Content-Language"
        , CI ByteString
    "Content-Type"
        , CI ByteString
    "Expires"
        , CI ByteString
    "Last-Modified"
        , CI ByteString
    "Pragma"
        ]

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

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

    -- | Simple HTTP methods as defined in <https://www.w3.org/TR/cors/#simple-method>
    --
    simpleMethods ∷ [HTTP.Method]
    simpleMethods :: [ByteString]
    simpleMethods =
        [ ByteString
    "GET"
        , ByteString
    "HEAD"
        , ByteString
    "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 :: ByteString -> ResponseHeaders -> Bool
    isSimple ByteString
    method ResponseHeaders
    headers
        = ByteString
    method ByteString -> [ByteString] -> Bool
    forall a. Eq a => a -> [a] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [ByteString]
    simpleMethods
        Bool -> Bool -> Bool
    ∧ (Header -> CI ByteString) -> ResponseHeaders -> [CI ByteString]
    forall a b. (a -> b) -> [a] -> [b]
    map Header -> CI ByteString
    forall a b. (a, b) -> a
    fst ResponseHeaders
    headers [CI ByteString] -> [CI ByteString] -> Bool
    forall α. Eq α => [α] -> [α] -> Bool
    `isSubsetOf` [CI ByteString]
    simpleHeaders
        Bool -> Bool -> Bool
    ∧ case (ByteString
    method, CI ByteString -> ResponseHeaders -> Maybe ByteString
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup CI ByteString
    "content-type" ResponseHeaders
    headers) of
            (ByteString
    "POST", Just ByteString
    x) → ByteString -> CI ByteString
    forall s. FoldCase s => s -> CI s
    CI.mk ByteString
    x CI ByteString -> [CI ByteString] -> Bool
    forall a. Eq a => a -> [a] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [CI ByteString]
    simpleContentTypes
            (ByteString, Maybe ByteString)
    _ → Bool
    True

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

    httpHeaderNameParser ∷ P.Parser HTTP.HeaderName
    httpHeaderNameParser :: Parser (CI ByteString)
    httpHeaderNameParser = [Char] -> CI ByteString
    forall a. IsString a => [Char] -> a
    fromString ([Char] -> CI ByteString)
    -> Parser ByteString [Char] -> Parser (CI ByteString)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Parser ByteString Char -> Parser ByteString [Char]
    forall (f :: * -> *) a. Alternative f => f a -> f [a]
    P.many1 ((Char -> Bool) -> Parser ByteString Char
    P.satisfy Char -> Bool
    isHttpHeaderNameChar) Parser (CI ByteString) -> [Char] -> Parser (CI ByteString)
    forall i a. Parser i a -> [Char] -> Parser i a
    P.<?> [Char]
    "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 [CI ByteString]
    httpHeaderNameListParser = Parser ByteString [Char]
    spaces Parser ByteString [Char]
    -> Parser [CI ByteString] -> Parser [CI ByteString]
    forall a b.
    Parser ByteString a -> Parser ByteString b -> Parser ByteString b
    forall (f :: * -> *) a b. Applicative f => f a -> f b -> f b
    *> Parser (CI ByteString)
    -> Parser ByteString Char -> Parser [CI ByteString]
    forall (f :: * -> *) a s. Alternative f => f a -> f s -> f [a]
    P.sepBy (Parser (CI ByteString)
    httpHeaderNameParser Parser (CI ByteString)
    -> Parser ByteString [Char] -> Parser (CI ByteString)
    forall a b.
    Parser ByteString a -> Parser ByteString b -> Parser ByteString a
    forall (f :: * -> *) a b. Applicative f => f a -> f b -> f a
    <* Parser ByteString [Char]
    spaces) (Char -> Parser ByteString Char
    P.char Char
    ',') Parser [CI ByteString]
    -> Parser ByteString [Char] -> Parser [CI ByteString]
    forall a b.
    Parser ByteString a -> Parser ByteString b -> Parser ByteString a
    forall (f :: * -> *) a b. Applicative f => f a -> f b -> f a
    <* Parser ByteString [Char]
    spaces
      where
        spaces :: Parser ByteString [Char]
    spaces = Parser ByteString Char -> Parser ByteString [Char]
    forall (m :: * -> *) a. MonadPlus m => m a -> m [a]
    P.many' Parser ByteString Char
    P.space

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

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

    -- | Add HTTP headers to a WAI response
    --
    addHeaders ∷ HTTP.ResponseHeaders → WAI.Middleware
    addHeaders :: ResponseHeaders -> Middleware
    addHeaders ResponseHeaders
    hdrs Application
    app Request
    req 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 → do
        let (Status
    st, ResponseHeaders
    headers, (StreamingBody -> IO ResponseReceived) -> IO ResponseReceived
    streamHandle) = Response
    -> (Status, ResponseHeaders,
        (StreamingBody -> IO ResponseReceived) -> IO ResponseReceived)
    forall a.
    Response
    -> (Status, ResponseHeaders, (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
    $ \StreamingBody
    streamBody →
            Response -> IO ResponseReceived
    respond (Response -> IO ResponseReceived)
    -> Response -> IO ResponseReceived
    forall a b. (a -> b) -> a -> b
    $ Status -> ResponseHeaders -> StreamingBody -> Response
    WAI.responseStream Status
    st (ResponseHeaders
    headers ResponseHeaders -> ResponseHeaders -> ResponseHeaders
    forall α. Monoid α => α -> α -> α
    ⊕ ResponseHeaders
    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 :: [CI ByteString] -> ByteString
    hdrLI [CI ByteString]
    l = ByteString -> [ByteString] -> ByteString
    B8.intercalate ByteString
    ", " ((CI ByteString -> ByteString) -> [CI ByteString] -> [ByteString]
    forall a b. (a -> b) -> [a] -> [b]
    map CI ByteString -> ByteString
    forall s. CI s -> s
    CI.original [CI ByteString]
    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 :: [ByteString] -> ByteString
    hdrL = ByteString -> [ByteString] -> ByteString
    B8.intercalate ByteString
    ", "

    corsFailure
        ∷ B8.ByteString -- ^ body
        → WAI.Response
    corsFailure :: ByteString -> Response
    corsFailure ByteString
    msg = Status -> ResponseHeaders -> ByteString -> Response
    WAI.responseLBS Status
    HTTP.status400 [(CI ByteString
    "Content-Type", ByteString
    "text/html; charset-utf-8")] (ByteString -> ByteString
    LB8.fromStrict ByteString
    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 Request
    req =
        (ByteString -> CI ByteString)
    -> Maybe ByteString -> Maybe (CI ByteString)
    forall a b. (a -> b) -> Maybe a -> Maybe b
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap ByteString -> CI ByteString
    forall s. FoldCase s => s -> CI s
    CI.mk (CI ByteString -> ResponseHeaders -> Maybe ByteString
    forall a b. Eq a => a -> [(a, b)] -> Maybe b
    lookup CI ByteString
    "upgrade" (ResponseHeaders -> Maybe ByteString)
    -> ResponseHeaders -> Maybe ByteString
    forall a b. (a -> b) -> a -> b
    $ Request -> ResponseHeaders
    WAI.requestHeaders Request
    req) Maybe (CI ByteString) -> Maybe (CI ByteString) -> Bool
    forall a. Eq a => a -> a -> Bool
    == CI ByteString -> Maybe (CI ByteString)
    forall a. a -> Maybe a
    Just CI ByteString
    "websocket"