[Haskell-beginners] database question

[Daniel Fischer]

On Saturday 18 September 2010 20:40:58, Gaius Hammond wrote:
> Hi all,
>
>
>
> I am trying to write the simplest possible database program in a
> monadic style. The aim of the program is to connect to the database,
> run a query, then print the results. My approach is
>
>
> - use the Reader monad to pass around the connection handle (lda) to
> the database
> - use the Writer monad to store the result set (rs) on its way back
> out - converted into [String] (one String per row)
>
>
>
> My code is:
>
>
>
> module Main where
>
> import Control.Monad.Reader
> import Control.Monad.Writer
> import Database.HDBC
> import Database.HDBC.Sqlite3
>
> runDbApp lda f =
>    do (a, rs) <- runWriterT (runReaderT f lda)
>       return rs
>
> doQuery::MonadIO m => ReaderT Connection (WriterT [String] m) ()
> doQuery = do
>    lda <- ask -- get the database handle from the Reader
>    rs <- quickQuery lda "select datetime ('now')" []
>
>    let rs' = map convRow rs
>    mapM_ tell rs' -- store the results in the Writer
>
>    where convRow [x] = (fromSql x)::String
>
> main = handleSqlError $ do
>    lda <- connectSqlite3 "test.db"
>    rs <- runDbApp lda $
>               doQuery
>
>    mapM_ putStrLn rs
>
> -- end of file
>
>
>
>
> And the error is
>
>
>
>
> home/gaius/Projects/MonadDb/MonadDb.hs:15:2:
>      Couldn't match expected type `IO [[SqlValue]]'
>             against inferred type `ReaderT
>                                      Connection (WriterT [String] m)
> [[SqlValue]]'
>      In a stmt of a 'do' expression:
>          rs <- quickQuery lda "select datetime ('now')" []
>
>
>
>
>
> 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.

> Any advice greatly appreciated. I have been struggling with this since
> lunchtime!
>

Hopefully it didn't stop you from eating.

Cheers,
Daniel