Hi Tom, I played a bit with your suggestion, and it is running now :-) But instead of IO [Int] I think we need IO [Only Int] because of the 1-element-tupel problem? With IO [Only Int] it looks like this: ------------------------------------------------------------------------------------------- {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple myconn :: ConnectInfo myconn = defaultConnectInfo { connectUser = "test", connectPassword = "test", connectDatabase = "test"} main :: IO () main = do c <- connect myconn :: IO Connection rs <- query_ c "select 2 + 2" :: IO [Only Int] putStrLn $ "Result from database " ++ show (fromOnly $ head rs) return () ------------------------------------------------------------------------------------------- Best regards Hartmut On 08/18/2013 12:11 AM, Tom Ellis wrote:
On Sat, Aug 17, 2013 at 11:59:24PM +0200, Hartmut Pfarr wrote:
{-# LANGUAGE OverloadedStrings #-}
import Database.PostgreSQL.Simple import Database.PostgreSQL.Simple.FromRow
hello :: (FromRow a) => IO [a] hello = do conn <- connect defaultConnectInfo query_ conn "select 2 + 2"
Either
main = print =<< (hello :: IO [Int])
or give hello a monomorphic type signature, such as
hello :: IO [Int]
Tom
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