refactor: Move each Wai Middleware to a separate file

src/PostgREST/Cors.hs

@@ -0,0 +1,42 @@
+{-|
+Module      : PostgREST.Cors
+Description : Wai Middleware to set cors policy.
+-}
+module PostgREST.Cors (middleware) where
+
+import qualified Data.ByteString.Char8       as BS
+import qualified Data.CaseInsensitive        as CI
+import qualified Network.Wai                 as Wai
+import qualified Network.Wai.Middleware.Cors as Wai
+
+import Data.List (lookup)
+
+import Protolude
+
+middleware :: Wai.Middleware
+middleware = Wai.cors corsPolicy
+
+-- | CORS policy to be used in by Wai Cors middleware
+corsPolicy :: Wai.Request -> Maybe Wai.CorsResourcePolicy
+corsPolicy req = case lookup "origin" headers of
+  Just origin ->
+    Just Wai.CorsResourcePolicy
+    { Wai.corsOrigins = Just ([origin], True)
+    , Wai.corsMethods = ["GET", "POST", "PATCH", "PUT", "DELETE", "OPTIONS"]
+    , Wai.corsRequestHeaders = "Authorization" : accHeaders
+    , Wai.corsExposedHeaders = Just
+      [ "Content-Encoding", "Content-Location", "Content-Range", "Content-Type"
+      , "Date", "Location", "Server", "Transfer-Encoding", "Range-Unit"]
+    , Wai.corsMaxAge = Just $ 60*60*24
+    , Wai.corsVaryOrigin = False
+    , Wai.corsRequireOrigin = False
+    , Wai.corsIgnoreFailures = True
+    }
+  Nothing -> Nothing
+  where
+    headers = Wai.requestHeaders req
+    accHeaders = case lookup "access-control-request-headers" headers of
+      Just hdrs -> map (CI.mk . BS.strip) $ BS.split ',' hdrs
+       -- Impossible case, Middleware.Cors will not evaluate this when
+       -- the Access-Control-Request-Headers header is not set.
+      Nothing   -> []