extensible-effectsの作用の拡大

upgrade.hs

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}

import Control.Eff
import Control.Monad.Free.Reflection
import Data.OpenUnion
import Data.Typeable

-- http://aiya000.github.io/posts/2017-08-22-my-experience-of-eff-convertion.html

upgrade :: (Functor t, Typeable t) => Eff r a -> Eff (t :> r) a
upgrade = fromView . go . toView
  where
    go (Impure uni) = Impure $ weaken (upgrade <$> uni)
    go (Pure x) = Pure x