On 18 Sep 2010, at 21:03, Daniel Fischer wrote:
Why does it think that that is the final expression of the function?
It doesn't, but you said the do-expression had type ReaderT Connection (WriterT [String] m) () while
quickQuery :: IConnection conn => conn -> String -> [SqlValue] -> IO [[SqlValue]]
The types don't match, hence the error. But you have a MonadIO constraint, so
rs <- liftIO $ quickQuery ...
should fix it.
Aha! It does indeed, thanks :-) I had assumed that with the rs <- quickQuery I was getting the [[SqlValue]] "out" of the IO Monad - but I have to lift it into the IO Monad first, since I am in a monadic context? This would be the non- monadic version module Main where import Database.HDBC import Database.HDBC.Sqlite3 main:: IO () main = do lda <- connectSqlite3 "test.db" rs <- quickQuery lda "select datetime ('now')" [] mapM_ putStrLn (map convRow rs) where convRow [x] = (fromSql x)::String -- EOF Oh, but I am already in the IO monad there aren't I.... And doQuery *isn't* but I have declared that it *should be* (e.g. an instance of typeclass MonadIO). Is that accurate?
Any advice greatly appreciated. I have been struggling with this since lunchtime!
Hopefully it didn't stop you from eating.
Nothing could do that :-) Cheers, G