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%3Acatch
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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