[Haskell-beginners] database question
daniel.is.fischer at web.de
Sat Sep 18 16:49:41 EDT 2010
On Saturday 18 September 2010 22:30:46, Gaius Hammond wrote:
> 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
Sort of, but only in a do-block of type IO something, you had it in a do-
block of type
ReaderT Connection (WriterT [String] m) something
There it doesn't type-check. Remember,
do a <- act
is syntactic sugar for
act >>= otherAct
The type of (>>=) says the monads on both sides must be the same.
> - but I have to lift it into the IO
> Monad first, since I am in a monadic context?
No, you have to lift the IO-action (quickQuery args) into the MonadIO
ReaderT Connection (WriterT [String] m),
liftIO :: MonadIO m => IO a -> m a
lifts IO-actions to another monad.
> 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.
Yup, in main, you're in IO.
> ... And doQuery
> *isn't* but I have declared that it *should be* (e.g. an instance of
> typeclass MonadIO). Is that accurate?
No, MonadIO says, you can run IO-actions from inside that monad, but you
have to lift them in order to run them (except the MonadIO you use is IO
itself). You said that you wanted to be able to do that, but forgot the
> >> Any advice greatly appreciated. I have been struggling with this
> >> since
> >> lunchtime!
> > Hopefully it didn't stop you from eating.
> Nothing could do that :-)
Glad to read that.
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