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

John Goerzen jgoerzen at complete.org
Mon Oct 9 18:57:12 EDT 2006

I forgot to add a couple of things.

First, you mentioned the unknown exception.  If you were to replace the
"main =" line with "main = handleSqlError $ ", then you would find a
more useful error message.  That will cause HDBC to automatically
transform any uncaught HDBC exception into a standard Haskell I/O error.
You can't do as much with it programmatically that way, but it will
autoamtically be displayed in a useful fashion.

Second, you could make sure you always get it right with regard to when
to disconnect.  For instance:

main = handleSqlError $ bracket (DB.connectODBC "DSN=test")

procdb dbh =
   do res <- DB.getTables dbh
      print (show ((concat . intersperse ", ") res))

bracket is defined in Control.Exception.  

While HDBC will automatically clean up connections when a HDBC
Connection is garbage-collected, this is probably the most correct way
to use it anyway, and is analogous to the example in the GHC library

   (openFile "filename" ReadMode)
   (\handle -> do { ... })

It will ensure that DB.disconnect is always called, either after your
code completes, or after an uncaught exception.

But moreover, you isolate all of your code that requires the Connection
within a single function (or functions that it calls).  The Connection
does not escape the bracket, so there is no danger you might use it
after it has been closed.  It may be a useful idiom for you if you are
struggling with synchronization issues.  You can also consider it for
use with HDBC Statements, regular disk files, etc.

Note that the following is NOT correct:

bracket (connectAndReadTables) (printTables) (disconnect)

-- John

On Mon, Oct 09, 2006 at 04:01:02PM -0600, Tim Smith wrote:
> "unknown exception".  If I use the results before disconnecting, it
> works fine.
> module Main
> where
> import Data.List (intersperse)
> import qualified Database.HDBC as DB
> import Database.HDBC.ODBC (connectODBC)
> main :: IO ()
> main =
>    do
>    dbh <- connectODBC "DSN=test"
>    res <- DB.getTables dbh
>    -- print (show ((concat . intersperse ", ") res))
>    DB.disconnect dbh
>    print (show ((concat . intersperse ", ") res))
> Compiling and running this will show:
> $ ./db-discon
> db-discon: unknown exception
> If I uncomment the first 'print' line, then it works as expected:
> $ ./db-discon
> "\"d1, foo, odbctest\""
> "\"d1, foo, odbctest\""
> Am I just expecting the wrong thing from Haskell?  Is there a
> technical reason why HDBC can't synchronize the IO so that everything
> is resolved before the disconnect?  Or is this a bug in HDBC?
> Thanks,
> Timothy
> -- 
> If you're not part of the solution, you're part of the precipitate.

John Goerzen
Author, Foundations of Python Network Programming

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