[Haskell-cafe] MonadBaseControl and Freer
Li-yao Xia
lysxia at gmail.com
Fri Feb 2 00:24:11 UTC 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:
>>>>>> "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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