Network.Wai.Middleware.Catch

Exception handling for Wai and Warp.

By default Warp not handles exceptions well. It just log them to console. This package - an attempt to solve the problem.

 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE ScopedTypeVariables #-} 

 import Data.Typeable (Typeable)
 import Control.Exception (Exception, throw)
 import Network.HTTP.Types
 import Network.Wai.Handler.Warp (run)
 import Data.ByteString.Lazy.Char8 (pack) -- Just for an orphan instance

 import Network.Wai
 import Network.Wai.Middleware.Catch

 data MyException = MyException String deriving (Show, Typeable)
 instance Exception MyException

 main :: IO ()
 main = do
     putStrLn $ "http://localhost:8888/"
     run 8888 $ (protect handler) $ app
     -- ... try 'protect'' and you see what /err/ request will 
     -- be handled with code 500

 app :: Application
 app req = case rawPathInfo req of
     "/ok" -> return $ responseLBS status200 
                       [("Content-Type", "text/plain")] "OK"
     "/err" -> error "Error"
     "/exc" -> throw $ MyException "Raised exception"
     _ -> return $ responseLBS status200 [("Content-Type", "text/plain")] 
             "Try any of /ok, /exc, /err"

 -- Our handler
 handler (e :: MyException) _ = return $ 
    responseLBS status200 [("Content-Type", "text/plain")] (pack $ show e)

The only drawback stems from the basic advantages of Haskell - laziness. All errors within Wai ResponseBuilder will not be caught. Thus, the following code will not work:

 ... return $ responseLBS undefined ...

To ensure catch all errors, you need to consume all data before feeding the builder.

Documentation

protect

Arguments

:: Exception e

=> (e -> Application)

Handler

-> Middleware

Handles exceptions in responses. If exception isn't handled - it will be rethrown further. To ensure handle all errors use protect'.

protect'

Arguments

:: Exception e

=> (e -> Application)

Handler

-> Middleware

Strict version of protect. Handles all exceptions. If exception not handled, this function return 500 - Internal Server Error with empty headers and body what contains show of error.