[Haskell-cafe] Database.postgreSQL.Simple - ambigious type

[Hartmut Pfarr]

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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