[Haskell-cafe] FastCGI error handling

3 Aug 2009

      I've been trying to write some simple web application in haskell using
FastCGI, HDBC and HStringTemplate. I've got stuck with the following
problem.

HDBC throws some exceptions and I wanted them to be caught and logged.
The code was:

-- Main.hs
module Main (main) where

...
import Control.Concurrent
import Network.FastCGI

driver :: CGI CGIResult
driver = ...

main :: IO ()
main = runFastCGIConcurrent' forkIO 10 (handleErrors test)

But all I've got on the page and in the log is "SomeException" string.

I've tried the workaround:
-- Main.hs
module Main (main) where

import Network.FastCGI
import Control.Exception
import System.IO

driver :: CGI CGIResult
driver = ...

...
main :: IO ()
main = runFastCGI driver `catch` \ex -> hPutStr stderr $ show
(ex::SomeException)

And this worked fine. I've got a detaied SqlException string in the log file.

So I've decided to update exception handling in CGI package and
written the following module:

-- Network/CGI/Monad/NewException.hs
module Network.CGI.Monad.NewException where

import Prelude hiding (catch)

import Control.Exception
import Control.Monad
import Control.Monad.Writer
import Control.Monad.Reader

import Network.CGI.Monad hiding (throwCGI, catchCGI, tryCGI)
import Network.CGI (outputInternalServerError)

-- | Throw an exception in a CGI monad. The monad is required to be
--   a 'MonadIO', so that we can use 'throwIO' to guarantee ordering.
throwCGI :: (MonadCGI m, MonadIO m, Exception e) => e -> m a
throwCGI = liftIO . throwIO

-- | Catches any expection thrown by a CGI action, and uses the given
--   exception handler if an exception is thrown.
catchCGI :: (Exception e) => CGI a -> (e -> CGI a) -> CGI a
catchCGI c h = tryCGI c >>= either h return

handleCGI :: (Exception e) => (e -> CGI a) -> CGI a -> CGI a
handleCGI = flip catchCGI

-- | Catches any exception thrown by an CGI action, and returns either
--   the exception, or if no exception was raised, the result of the action.
tryCGI :: (Exception e) => CGI a -> CGI (Either e a)
tryCGI (CGIT c) = CGIT (ReaderT (\r -> WriterT (f (runWriterT
(runReaderT c r)))))
    where
      f = liftM (either (\ex -> (Left ex,mempty)) (\(a,w) -> (Right a,w))) . try

handleErrors = handleCGI outputException

outputException e = outputInternalServerError [show (e :: SomeException)]

Then I've updated Main.hs accordingly:

module Main (main) where

...
import Control.Concurrent
import Network.FastCGI hiding (throwCGI, catchCGI, tryCGI,
handleErrors, outputException)
import Network.CGI.Monad.NewException

driver :: CGI CGIResult
driver = ...

main :: IO ()
-- main = runFastCGIConcurrent' forkIO 10 (handleErrors test) -- This
allways gives internal error without a chance to find out what happens
main = runFastCGI $ handleErrors test -- This works fine

So one variant of main function works, the other allways gives no
output. What can be the case?

-- 
Victor Nazarov