[Haskell-cafe] Re: Takusen - error handling and DBM monad

Paul Moore wrote:

Now, I can protect my query with something like

main = do withSession (connect "..." "..." "...") ( do catchDB (do -- simple query, returning reversed list of rows. r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ putStrLn $ show r ) catcher )

So far, so good. The syntax is messy, but a little refactoring should fix that.

But this doesn't catch errors in the connect call.

Wrapping the withSession in catchDB doesn't work, as withSession is in the IO monad [1], not the DBM one. But using catch with catcher doesn't work, as that expects an error handler in the IO monad, but catcher is in the DBM monad.

You can catch exceptions in the IO monad using 'catch' from the prelude: http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Acatc... Unfortunately, you will have to write another error handler though, because the catch expects the handler to have type (IOError -> IO a) instead of (DBException -> DBM ...). Using both catch and catchDB should let you handle all the errors. BTW, what does withSession return if there is a DBException?