module Network.Wai.Middleware.AcceptOverride
    ( -- $howto
      acceptOverride
    ) where

import Network.Wai
import Control.Monad (join)

import Network.Wai.Header (replaceHeader)

-- $howto
-- This 'Middleware' provides a way for the request itself to
-- tell the server to override the \"Accept\" header by looking
-- for the \"_accept\" query parameter in the query string and
-- inserting or replacing the \"Accept\" header with that string.
--
-- For example:
--
-- @
-- ?_accept=SomeValue
-- @
--
-- This will result in \"Accept: SomeValue\" being set in the
-- request as a header, and all other previous \"Accept\" headers
-- will be removed from the request.

acceptOverride :: Middleware
acceptOverride :: Middleware
acceptOverride Application
app Request
req =
    Application
app Request
req'
  where
    req' :: Request
req' =
        case forall (m :: * -> *) a. Monad m => m (m a) -> m a
join forall a b. (a -> b) -> a -> b
$ forall a b. Eq a => a -> [(a, b)] -> Maybe b
lookup ByteString
"_accept" forall a b. (a -> b) -> a -> b
$ Request -> Query
queryString Request
req of
            Maybe ByteString
Nothing -> Request
req
            Just ByteString
a -> Request
req {
                requestHeaders :: RequestHeaders
requestHeaders = HeaderName -> ByteString -> RequestHeaders -> RequestHeaders
replaceHeader HeaderName
"Accept" ByteString
a forall a b. (a -> b) -> a -> b
$ Request -> RequestHeaders
requestHeaders Request
req
              }