Hello, To implement high-performance web servers, I would like to remove overhead of String and want to use ByteString instead. Unfortunately, WAI is using FilePath (String) in ResponseFile. ResponseFile Status ResponseHeaders FilePath (Maybe FilePart) To my experience, the third argument is created from rawPathInfo (ByteString) in many cases. So, using FilePath is overhead. Since new Haskell Platform is released, I think I can implement new simple-sendfile which provides Network.Sendfile.ByteString: sendfile :: Socket -> RawFilePath -> FileRange -> IO () -> IO () Note that RawFilePath is a type synonym of ByteString. What do you think of this? A1) It's too late. A2) We can change FilePath to RawFilePath in ReponseFile. A3) We can provide a new constructor: ReponseRawFile A4) Others --Kazu