[Haskell-cafe] MonadBaseControl and Freer

[Li-yao Xia]

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:
>>>>>> "LB" == Lana Black <lanablack at amok.cc> 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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