{-# OPTIONS_GHC -XEmptyDataDecls #-}
--
-- (c) 2009, Sigbjorn Finne.
-- 
-- Example of how to create and execute LINQ queries
-- from Haskell.
--
-- To test, call 'whereQuery' from GHCi.
--
module LinqQuery where

import Data.Int
import NET
import NET.System.Collections.Generic.Queue ( Queue, enqueue )
import NET.System.Collections.Generic.List as List ( get_Item )
import NET.System.Linq.Enumerable ( whereQ, toList )

newQueue :: NET.Type a => a -> IO (Queue () a)
newQueue a = newGeneric "System.Collections.Generic.Queue" a

whereQuery :: IO ()
whereQuery = do
  intQ <- newQueue (0::Int32)
  mapM_ (flip enqueue intQ) [1..5]

  let queryPred v = do
        putStrLn ("Predicate passed argument: " ++ show v)
	return (v>=3)

  putStrLn "Performing the query..."
   -- creating the LINQ query, passing it the (Haskell) predicate
  res <- intQ # whereQ queryPred
  putStrLn "..query done; predicate not yet invoked.."
   -- let's force the query by getting out a result list..
  ls    <- res # getList 
  putStrLn ("Query result: " ++ show ls)

getList :: (Type a, Result a) => Queue () a -> IO [a]
getList q = do
 ls    <- q # toList
 count <- ls # getField "Count" ()
 mapM (\ i -> ls # List.get_Item i) [(0::Int32)..(count-1)]