[Haskell-cafe] Strictness, order of IO operations: NewCGI & HDBC

10 Aug 2006

      Hello!  I'm having a problem with using NewCGI & HDBC together.

NewCGI:
 http://www.cs.chalmers.se/~bringert/darcs/haskell-cgi/doc/
HDBC:
 http://quux.org/devel/hdbc
 http://darcs.complete.org/hdbc/

I've distilled it down to a small test case:

\begin{code}
module Main
where

import Data.List (intersperse)
import qualified Database.HDBC as DB
import Database.HDBC.ODBC (connectODBC)
import Network.NewCGI

main :: IO ()
main = runCGI (handleErrors cgiMain)

cgiMain :: CGI CGIResult
cgiMain =
       do
       dbh <- liftIO $ connectODBC "DSN=test"
       res <- liftIO (DB.getTables dbh)
       -- Remove the disconnect call, and all works
       liftIO (DB.disconnect dbh)
       output ((concat . intersperse ", ") res)
\end{code}

2:46 ~/m/tmp/hs$ ./cgidb
Content-type: text/html; charset=ISO-8859-1

cgidb: unknown exception

If I comment out the DB.disconnect call, then it works fine.

I tried to work around it with the DeepSeq module, but couldn't find
an application of $!! or deepSeq which would make it function.

I made a similar test which uses file IO instead of HDBC, and it
behaves the way I want:

\begin{code}
module Main
where

import Network.NewCGI
import System.IO

main :: IO ()
main = runCGI (handleErrors cgiMain)

cgiMain :: CGI CGIResult
cgiMain =
       do
       h <- liftIO $ openFile "t.hs" ReadMode
       res <- liftIO (hGetLine h)
       liftIO (hClose h)
       output res
\end{code}

2:55 ~/m/tmp/hs$ ./t
Content-type: text/html; charset=ISO-8859-1

module Main

My guess is that I'm doing something wrong, but I'm not sure what it
is.  Or, is this a bug in HDBC, somehow?

Thanks for any ideas you might have,

Tim
-- 
If you're not part of the solution, you're part of the precipitate.

[Haskell-cafe] Strictness, order of IO operations: NewCGI & HDBC

Tim Smith