Copyright 2010-2012 Cognimeta Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
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{-# LANGUAGE TupleSections, FlexibleInstances, MultiParamTypeClasses #-}

module Cgm.Control.Monad.State(
  module Control.Monad.State.Class,
  module Control.Monad.Trans.Class,
  module Control.Monad.IO.Class
  ) where

import Control.Applicative
import Control.Monad
import Control.Arrow
import Data.Functor.Identity
import Data.Maybe
import Control.Monad.State.Class
import Control.Monad.Trans.Class
import Control.Monad.IO.Class
import Control.Concurrent.MVar
import qualified Control.Monad.State.Strict as Std

import Data.Lens hiding (focus)
import Control.Comonad.Trans.Store

-- | runStateT and runState do not have the usual types: for now we do not make it too easy to discard the precious 'Nothing'
newtype StateT s m a = StateT {runStateT :: s -> m (a, Maybe s)}

type State s = StateT s Identity
mState :: (s -> (a, Maybe s)) -> State s a 
mState = StateT . fmap Identity
runState :: State s a -> s -> (a, Maybe s)
runState = fmap runIdentity . runStateT

instance Functor m => Functor (StateT s m) where
  fmap f = StateT . fmap (fmap $ first f) . runStateT
instance (Functor m, Monad m) => Applicative (StateT s m) where  
  pure = return
  (<*>) = ap
instance Monad m => Monad (StateT s m) where
  return = lift . return
  (StateT c) >>= fd = StateT $ \s -> 
    c s >>= \(a, ms) ->   -- We are strict in the state. We do as in Control.Monad.Trans.State.Strict (not .Lazy)
    let d = runStateT (fd a) 
    in maybe (d s) (\s' -> liftM (second $ Just . fromMaybe s') $ d s') ms
instance MonadTrans (StateT s) where
  lift = StateT . const . liftM (, Nothing)
instance MonadIO m => MonadIO (StateT s m) where
  liftIO = lift . liftIO
instance Monad m => MonadState s (StateT s m) where
  get = StateT $ return . (, Nothing)
  put = StateT . const . return . ((), ) . Just

focus :: Monad m => Lens t s -> StateT s m a -> StateT t m a
focus l (StateT smas) = StateT $ (\(st, s) -> liftM (second $ fmap st) $ smas s) . runStore . runLens l

-- | functions st and ts should form a bijection since a StateT t with no changes will become a StateT s with no changes, no matter what st and ts are
viewState :: Monad m => (s -> t, t -> s) -> StateT t m a -> StateT s m a
viewState (st, ts) (StateT tf) = StateT $ liftM (second $ fmap ts) . tf . st

-- Work on a part of the state, that may not exist for some inputs
partialState :: Monad m => m a -> (s -> Maybe t, t -> s) -> StateT t m a -> StateT s m a
--partialState d (smt, ts) (StateT tf) = get >>= \s -> maybe (lift d) ((>>= \(a, mt) -> maybe (return ()) (put . ts) mt >> return a) . lift . tf) $ smt s
partialState d (smt, ts) t = viewState (\s -> maybe (Left s) Right $ smt s, either id ts) $ eitherState (lift d) t -- note that we use a view 's -> Either s t'

-- Like partialState, but remembers which path was taken
partialStateE :: Monad m => m a -> (s -> Maybe t, t -> s) -> StateT t m b -> StateT s m (Either a b)
partialStateE d fs t = partialState (liftM Left d) fs (liftM Right t)

eitherState :: Monad m => StateT s m a -> StateT t m a -> StateT (Either s t) m a
eitherState (StateT sf) (StateT tf) = StateT $ either (liftM (second $ fmap Left) . sf) (liftM (second $ fmap Right) . tf)
maybeState :: Monad m => m a -> StateT s m a -> StateT (Maybe s) m a
maybeState n s = viewState (maybe (Left ()) Right, either (const Nothing) Just) $ eitherState (lift n) s
toStandardState :: Monad m => StateT s m a -> Std.StateT s m a
toStandardState (StateT f) = Std.StateT $ \s -> liftM (second $ fromMaybe s) $ f s

mapStateT :: (m (a, Maybe s) -> n (b, Maybe s)) -> StateT s m a -> StateT s n b
mapStateT f m = StateT $ f . runStateT m

pairStateT :: Functor m => StateT s (StateT t m) a -> StateT (s, t) m a
pairStateT (StateT sf) = StateT $ \(s, t) -> fmap (\((a, ms), mt) -> (a, combineMaybes s t ms mt)) $ runStateT (sf s) t where
  combineMaybes s t ms = maybe (fmap (, t) ms) (Just . (fromMaybe s ms,))