[Haskell-cafe] Handling Postgresql array types
Boris Lykah
lykahb at gmail.com
Sat Dec 27 00:48:13 UTC 2014
Groundhog has support for PostgreSQL arrays and almost all operations on
them. Check out the docs at
http://hackage.haskell.org/package/groundhog-postgresql/docs/Database-Groundhog-Postgresql-Array.html
Here is a tutorial for the library
https://www.fpcomplete.com/user/lykahb/groundhog
The array code looks like
main = withPostgresqlConn connectionString . runDbConn $ do
key <- insert $ MyData $ Array [1, 2, 3]
results <- select $ arrayLength MyField 1 <. (5 :: Int) -- length of
array dimension 1 is less than 5
liftIO $ print results
On Fri, Dec 26, 2014 at 6:02 AM, Tom Ellis <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> On Fri, Dec 26, 2014 at 10:52:46AM +0000, Tom Ellis wrote:
> > On Fri, Dec 26, 2014 at 02:36:07PM +1100, Riaan wrote:
> > > So went back to HDBC. But now my queries are starting to get fairly
> long
> > > and I have been looking at libraries like Persistent with Esqualeto,
> > > HaskellDB and Groundhog to make my queries a little more composable and
> > > type safe.
> >
> > Don't forget to look at Opaleye too :) It doesn't really have much
> support
> > for arrays at the moment, but if you let me know what you want to do I'm
> > happy to help you.
> >
> > > So to my question. Does anyone have experience with one of these
> libraries
> > > in dealing with postgresql arrays
> >
> > postgresql-simple has
> >
> > instance (FromField a, Typeable a) => FromField (PGArray a)
> >
> > Does that not do exactly what you want?
>
> import Database.PostgreSQL.Simple (query_, ConnectInfo(..), connect, Only)
> import Database.PostgreSQL.Simple.Types (PGArray)
>
> import Data.String (fromString)
>
> connectInfo :: ConnectInfo
> connectInfo = ConnectInfo { connectHost = "localhost"
> , connectPort = 25433
> , connectUser = "tom"
> , connectPassword = "tom"
> , connectDatabase = "opaleye_test" }
>
> arrayQuery :: String
> arrayQuery = "select '{{1,2}, {3,4}}' :: integer[][]"
>
> main :: IO ()
> main = do
> conn <- connect connectInfo
> results <- query_ conn (fromString arrayQuery) :: IO [Only (PGArray
> (PGArray Int))]
> print results
>
> -- Output
> --
> -- ghci> main
> -- [Only {fromOnly = PGArray {fromPGArray = [PGArray {fromPGArray =
> [1,2]},PGArray {fromPGArray = [3,4]}]}}]
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--
Regards,
Boris
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