Re: [Haskell-cafe] MonadBaseControl and Freer

1 Feb 2018

      Since the MonadBase superclass is in the way, you could define a custom 
(EffMonadBaseControl m r) with no superclass (or at least not that one) 
and then

instance (MonadBase m (Eff r), EffMonadBaseControl m r)
   => MonadBaseControl m (Eff r)

(Here I also factored out Eff because why not.)

Another way may be to have MonadBase instances for Eff to follow the 
same structure of going through one effect at a time, instead of jumping 
to the last element directly via the OpenUnion API as freer-simple does.

On 02/01/2018 06:08 PM, John Wiegley wrote:
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"LB" == Lana Black  writes:
LB> As far as I understand, MonadBaseControl class does the following:
LB> captures the current state. Performs the action passed to lifeBaseWith or
LB> other wrapper functions. Returns the result wrapped in the captured state.
LB> Here's the instance I could come up with.
LB> instance (MonadBase m (Eff r), Typeable m, SetMember Lift (Lift m) r) =>
LB> MonadBaseControl m (Eff r) where
LB>     type StM (Eff r) a = Eff r a
LB>     liftBaseWith f = lift (f return)
LB> restoreM = id
LB> It obviously doesn't work, but I currently have no idea how to fix it,
LB> because `Eff r a' contains the state that needs to be captured and cannot
LB> be decomposed without losing data as far as I can see.
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
--
John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2
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Re: [Haskell-cafe] MonadBaseControl and Freer

Li-yao Xia