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