Thx, I changed now from query to query_ Now the coding is like that: ------------------------------------------------------------------------ {-# 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" main = return () ------------------------------------------------------------------------ I've no errors any more. But: I don't see any result (for sure, it is not coeded yet) I need some help to get data from "hello" via "FromRow" into the main function. E.g. I want to put the "hello" database result (the number "4") to the screen. Could anybody give an advice how I can accomplish this? Kind regards Hartmut On 08/17/2013 07:53 PM, Brandon Allbery wrote:

On Sat, Aug 17, 2013 at 1:35 PM, Hartmut Pfarr mailto:hartmut0407@googlemail.com> wrote:

(The example is identical to the first 5-liner-example in the package documentation)

As I read it, the example has a typo: it should be using `query_` instead of `query`. See http://hackage.haskell.org/packages/archive/postgresql-simple/0.3.5.0/doc/ht... for detals.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com mailto:allbery.b@gmail.com ballbery@sinenomine.net mailto:ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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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On Sat, Aug 17, 2013 at 5:59 PM, Hartmut Pfarr wrote:

query_ conn "select 2 + 2"

I've no errors any more. But: I don't see any result (for sure, it is not coeded yet)

Yes, because you're not capturing it; it's the return value from `query_`, which you are throwing away above instead of capturing with some kind of `res <- query_ ...`. Again, see that section of the documentation I pointed to for how to get results. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net