gistfile1.hs

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleInstances #-}

                                                                                                  
import Control.Applicative                                                                        
import Control.Monad                                                                              
import Control.Monad.IO.Class (liftIO)                                                            
import Database.PostgreSQL.Simple                                                                 
import Database.PostgreSQL.Simple.FromRow                                                         
import Database.PostgreSQL.Simple.Types (Null(..))
import Data.Word                    
import qualified Data.Text as T                                                                   
                                                                                                  
connInfo = ConnectInfo "localhost" (15432 :: Word16) "schoolobjects" "schoolobjects" "schoolobjects"                                                                                                
                                                                                                  
data Course = Course { title :: T.Text                                                            
                     , description :: Maybe T.Text                                                
                     , creditid :: Maybe Int                                                      
                     , status :: T.Text                                                           
                     } deriving Show                                                              
                                                                                                  
data Activity = Activity { title' :: T.Text                                                        
                         , courseid' :: Int                                                        
                         , status' :: T.Text                                                       
                         } deriving Show                                                          
                                                                                                  
instance FromRow Course where                                                                     
  fromRow = Course <$> field <*> field <*> field <*> field                                        
                                                                                                  
instance FromRow Activity where                                                                   
  fromRow = Activity <$> field <*> field <*> field                                       

instance FromRow (Maybe Activity) where
  fromRow = (null *> null *> null *> pure Nothing) <|> (Just <$> fromRow)
    where null = field :: RowParser Null            

main :: IO ()                                                                                     
main = do                                                                                         
  conn <- connect connInfo                                                                        
  c <- query_ conn "SELECT c.title,\
                        \  c.description,\
                        \  c.creditid,\
                        \  c.status,\
                        \  a.title,\
                        \  a.courseid,\
                        \  a.status\
                  \ FROM course c\
                  \ LEFT JOIN activity a on a.courseid = c.id\
                  \ limit 10" :: (IO [Course :. Maybe Activity])
  putStrLn $ show c