{-# LANGUAGE PackageImports #-}

module EasyPool
    ( EasyPool
    , withEasyPool
    , newEasyPool
    ) where

import Pool
import Control.Monad.STM
import Control.Concurrent.STM.TVar
import Control.Monad.IO.Class
import "MonadCatchIO-transformers" Control.Monad.CatchIO

data EasyPool r m = EasyPool
    { epPool :: Pool r
    , epMake :: m (Maybe r)
    }

withEasyPool :: MonadCatchIO m => EasyPool r m -> (r -> m a) -> m (Maybe a)
withEasyPool (EasyPool pool mk) = withPool pool mk

newEasyPool :: MonadCatchIO m => Int -> m r -> m (EasyPool r m)
newEasyPool count mk = do
    pool <- new
    texist <- liftIO $ newTVarIO 0
    return $ EasyPool pool $ mk' texist
  where
    mk' texist = do
        exist <- liftIO $ atomically $ readTVar texist
        if exist >= count
            then return Nothing
            else do
                r <- mk
                liftIO $ atomically $ do
                    exist <- readTVar texist
                    if exist >= count
                        then return Nothing
                        else return $ Just r