[Haskell-beginners] database question

Gaius Hammond gaius at gaius.org.uk
Sat Sep 18 14:40:58 EDT 2010

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 $

   mapM_ putStrLn rs

-- end of file

And the error is

     Couldn't match expected type `IO [[SqlValue]]'
            against inferred type `ReaderT
                                     Connection (WriterT [String] m)  
     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?  
Any advice greatly appreciated. I have been struggling with this since  



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