Demonstrating how to use the Haxl library.

Main.hs

{-# LANGUAGE DeriveDataTypeable, GADTs, MultiParamTypeClasses, OverloadedStrings, StandaloneDeriving, TypeFamilies #-}
import Control.Monad
import Data.Hashable
import Data.Typeable
import Haxl.Core


data DeepThought a where
    AnswerToLifeTheUniverseAndEverything :: DeepThought Int
  deriving Typeable
deriving instance Show (DeepThought a)
deriving instance Eq (DeepThought a)

runDeepThought :: DeepThought a -> ResultVar a -> IO ()
runDeepThought AnswerToLifeTheUniverseAndEverything var = putSuccess var 42

instance DataSourceName DeepThought where
    dataSourceName _ = "DeepThought"

instance Show1 DeepThought where
    show1 AnswerToLifeTheUniverseAndEverything = "AnswerToLifeTheUniverseAndEverything"

instance Hashable (DeepThought a) where
    hashWithSalt salt AnswerToLifeTheUniverseAndEverything = hashWithSalt salt ()

instance StateKey DeepThought where
    data State DeepThought = NoState

instance DataSource () DeepThought where
    fetch _ _ _ reqs = SyncFetch $ do
        forM_ reqs $ \(BlockedFetch req var) -> runDeepThought req var


initialState :: StateStore
initialState = stateSet NoState stateEmpty

-- |
-- >>> main
-- 42
main :: IO ()
main = do
    myEnv <- initEnv initialState ()
    r <- runHaxl myEnv (dataFetch AnswerToLifeTheUniverseAndEverything)
    print r