[Haskell-cafe] Re: Takusen - error handling and DBM monad
afalloon at synopsys.com
Wed Jan 31 13:43:48 EST 2007
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
> 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 , 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:
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?
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