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Bypass Coercible Type for Deriving Monad* Classes of New Transformers Composed of Transformers #149

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Bypass Coercible Type for Deriving Monad* Classes of New Transformers Composed of Transformers #149

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4f3d5d8
initial concept of Bypass for deriving via
BebeSparkelSparkel Apr 17, 2024
cad092e
switched to Lifting* convention
BebeSparkelSparkel Apr 18, 2024
f92ef90
added LiftingReader LiftingWriter LiftingState
BebeSparkelSparkel May 5, 2024
9e325fd
MonadTrans for LiftingState instance
BebeSparkelSparkel May 7, 2024
570366d
ghci panic
BebeSparkelSparkel May 7, 2024
72a6089
using mapping functions for LiftingWriter
BebeSparkelSparkel May 15, 2024
a40942d
using mapping functions for LiftingReader
BebeSparkelSparkel May 15, 2024
6a88894
fixed comment
BebeSparkelSparkel May 16, 2024
eca747d
fixed exports, added doc comments
BebeSparkelSparkel May 17, 2024
6e75a30
added example for LiftingReader and LiftingState
BebeSparkelSparkel May 20, 2024
b701e12
Swithced to using a polykinded type family LiftingWriter. Allows for …
BebeSparkelSparkel May 20, 2024
222f6f9
fixed reader and state type variables
BebeSparkelSparkel Jul 25, 2024
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2 changes: 2 additions & 0 deletions Control/Monad/Reader.hs
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Expand Up @@ -40,6 +40,8 @@ module Control.Monad.Reader (
-- * MonadReader class
MonadReader.MonadReader(..),
MonadReader.asks,
-- * Lifting helper type
MonadReader.LiftingReader,
-- * The Reader monad
Reader,
runReader,
Expand Down
22 changes: 20 additions & 2 deletions Control/Monad/Reader/Class.hs
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@@ -1,9 +1,11 @@
{-# LANGUAGE Safe #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
-- Search for UndecidableInstances to see why this is needed
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE Trustworthy #-}
-- Needed because the CPSed versions of Writer and State are secretly State
-- wrappers, which don't force such constraints, even though they should legally
-- be there.
Expand Down Expand Up @@ -48,6 +50,7 @@ than using the 'Control.Monad.State.State' monad.
module Control.Monad.Reader.Class (
MonadReader(..),
asks,
LiftingReader(..),
) where

import qualified Control.Monad.Trans.Cont as Cont
Expand All @@ -68,7 +71,8 @@ import qualified Control.Monad.Trans.Accum as Accum
import Control.Monad.Trans.Select (SelectT (SelectT), runSelectT)
import qualified Control.Monad.Trans.RWS.CPS as CPSRWS
import qualified Control.Monad.Trans.Writer.CPS as CPS
import Control.Monad.Trans.Class (lift)
import Control.Monad.Trans.Class (MonadTrans(lift))
import Data.Kind (Type)

-- ----------------------------------------------------------------------------
-- class MonadReader
Expand Down Expand Up @@ -202,3 +206,17 @@ instance
r <- ask
local f (runSelectT m (local (const r) . c))
reader = lift . reader

type LiftingReader :: ((Type -> Type) -> Type -> Type) -> (Type -> Type) -> Type -> Type
newtype LiftingReader t m a = LiftingReader (t m a)
deriving (Functor, Applicative, Monad, MonadTrans)

instance MonadReader r m => MonadReader r (LiftingReader (ReaderT r') m) where
ask = lift ask
local f (LiftingReader (ReaderT.ReaderT x)) = LiftingReader . ReaderT.ReaderT $ local f . x
reader = lift . reader

instance (MonadReader r m, Monoid w) => MonadReader r (LiftingReader (LazyRWS.RWST r' w s) m) where
ask = lift ask
local f (LiftingReader (LazyRWS.RWST x)) = LiftingReader . LazyRWS.RWST $ \r s -> local f $ x r s
reader = lift . reader