extensible_sample.hs

extensible_sample.hs

{-# LANGUAGE DataKinds        #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators    #-}

import Control.Monad.Reader
import Control.Monad.Reader.Class
import Data.Extensible
import Data.Extensible.Effect
import Data.Extensible.Effect.Default

-- ===================== mtl compatible style
-- ex_reader :: Reader Int Int             -- > NG
ex_reader :: (MonadReader Int m) => m Int  -- > OK
ex_reader = do
  x <- ask
  return $ x * 2

run_reader x = runReader ex_reader x -- normal

run_ex_reader x = leaveEff $ runReaderDef ex_reader x -- extensible 

-- extensible 1
ex_reader_ext :: ((Associate "foo" (ReaderEff Int)) xs ) => Eff xs Int
ex_reader_ext = do
  x <- askEff #foo
  return $ x * 2

run_ex_reader_ext x = leaveEff $ runReaderEff @ "foo" ex_reader_ext x -- TypeApplication

-- extensible 2
ex_reader_ext' :: Eff '["foo" >: ReaderEff Int] Int -- DataKinds, TypeOperators
ex_reader_ext' = do
  x <- askEff #foo
  return $ x * 2

run_ex_reader_ext' x = leaveEff $ runReaderEff ex_reader_ext' x