Defines the interface implemented by all web applications.

Example application:

 module Main where

 import qualified Data.ByteString as S
 import qualified Data.ByteString.Char8 as C (pack, unpack)
 import Network.Wai (Application(..), Enumerator(..))
 import System.Directory (getCurrentDirectory)
 import System.FilePath ((</>), makeRelative)
 import System.IO

 sendFile :: FilePath -> IO Enumerator
 sendFile fname = do
   cwd <- getCurrentDirectory
   h <- openBinaryFile (cwd </> makeRelative "/" fname) ReadMode
   let yieldBlock f z = do
              block <- S.hGetNonBlocking h 1024
              if S.null block then hClose h >> return z
                else do
                  z' <- f z block
                  case z' of
                    Left z''  -> hClose h >> return z''
                    Right z'' -> yieldBlock f z''
   return yieldBlock

 fileServer :: Application
 fileServer environ = do
   -- Here you should add security checks, etc.
   let contentType = (C.pack "Content-Type",
                      C.pack "application/octet-stream")
   enumerator <- sendFile $ C.unpack $ pathInfo environ
   return (200, pack "OK", [contentType], enumerator)