Re: [Haskell-cafe] MonadBaseControl and Freer

2 Feb 2018


      On 01/02/18 23:08, John Wiegley wrote:
...
Hmmm... all these type classes are getting in my way. I thought I had a good
start here, but it's proving hard to use. Maybe others have an idea how to
continue.
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
module FreerQuestion where
import Control.Monad.Base
import Control.Monad.Freer
import Control.Monad.Freer.Internal
import Control.Monad.Freer.Reader
import Control.Monad.Freer.State
import Control.Monad.Trans.Class
import Control.Monad.Trans.Control
import Data.OpenUnion
import Data.OpenUnion.Internal
instance MonadBase m m => MonadBaseControl m (Eff '[m]) where
     type StM (Eff '[m]) a = a
     liftBaseWith f = sendM (f runM)
     restoreM = return
instance (Data.OpenUnion.LastMember x (r ': s),
           Data.OpenUnion.Internal.FindElem x s (Reader e : r : s),
           MonadBase m x,
           MonadBaseControl m (Eff (r ': s)))
       => MonadBaseControl m (Eff (Reader e ': r : s)) where
     type StM (Eff (Reader e ': r ': s)) a = StM (Eff (r ': s)) a
     liftBaseWith f = do
         e <- ask
         raise $ liftBaseWith $ \runInBase ->
             f $ \k -> runInBase (runReader e k)
     restoreM = raise . restoreM
instance (Data.OpenUnion.LastMember x (r ': s),
           MonadBase m x,
           Data.OpenUnion.Internal.FindElem x s (State e : r : s),
           MonadBaseControl m (Eff (r ': s)))
       => MonadBaseControl m (Eff (State e ': r : s)) where
     type StM (Eff (State e ': r ': s)) a = StM (Eff (r ': s)) (a, e)
     liftBaseWith f = do
         e <- get @e
         raise $ liftBaseWith $ \runInBase ->
             f $ \k -> runInBase (runState e k)
     restoreM x = do
         (a, e :: e) <- raise (restoreM x)
         put e
         return a
foo :: (Member (Reader Int) r, Member (State Int) r, Member IO r) => Eff r ()
foo = do
     r <- ask @Int
     put @Int 1000
     () <- control $ \runInBase -> do
         putStrLn "In IO!"
         s' <- runInBase $ do
             put @Int 2000
         putStrLn "Back in IO!"
         return s'
     s <- get @Int
     send @IO $ print s
main :: IO ()
main = runM . evalState (200 :: Int) . runReader (10 :: Int) $ foo
Writing a separate instance for each effect didn't occur to me for some 
reason. Thank you! I'll try to follow this path.

Re: [Haskell-cafe] MonadBaseControl and Freer

Lana Black