[Haskell-cafe] Handling Postgresql array types

[Boris Lykah]

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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