content: {-# LANGUAGE FlexibleInstances #-}
  {-# LANGUAGE TupleSections     #-}
  {- |
     Module      : Text.Pandoc.Readers.ODT.Arrows.State
     Copyright   : Copyright (C) 2015 Martin Linnemann
     License     : GNU GPL, version 2 or above
  
     Maintainer  : Martin Linnemann <theCodingMarlin@googlemail.com>
     Stability   : alpha
     Portability : portable
  
  An arrow that transports a state. It is in essence a more powerful version of
  the standard state monad. As it is such a simple extension, there are
  other version out there that do exactly the same.
  The implementation is duplicated, though, to add some useful features.
  Most of these might be implemented without access to innards, but it's much
  faster and easier to implement this way.
  -}
  
  module Text.Pandoc.Readers.ODT.Arrows.State
    ( ArrowState(..)
    , withState
    , modifyState
    , ignoringState
    , fromState
    , extractFromState
    , tryModifyState
    , withSubStateF
    , withSubStateF'
    , foldS
    , iterateS
    , iterateSL
    , iterateS'
    ) where
  
  import Control.Arrow
  import qualified Control.Category as Cat
  import Control.Monad
  import Data.List (foldl')
  import Text.Pandoc.Readers.ODT.Arrows.Utils
  import Text.Pandoc.Readers.ODT.Generic.Fallible
  
  
  newtype ArrowState state a b = ArrowState
    { runArrowState :: (state, a) -> (state, b) }
  
  -- | Constructor
  withState           :: (state -> a -> (state, b)) -> ArrowState state a b
  withState            = ArrowState . uncurry
  
  -- | Constructor
  modifyState         :: (state      ->  state    ) -> ArrowState state a a
  modifyState          = ArrowState . first
  
  -- | Constructor
  ignoringState       :: (         a ->         b ) -> ArrowState state a b
  ignoringState        = ArrowState . second
  
  -- | Constructor
  fromState           :: (state      -> (state, b)) -> ArrowState state a b
  fromState            = ArrowState . (.fst)
  
  -- | Constructor
  extractFromState    :: (state      ->         b ) -> ArrowState state x b
  extractFromState   f = ArrowState $ \(state,_) -> (state, f state)
  
  -- | Constructor
  tryModifyState      :: (state ->  Either f state)
                      -> ArrowState state a (Either f a)
  tryModifyState     f = ArrowState $ \(state,a)
                                    -> (state,).Left ||| (,Right a) $ f state
  
  instance Cat.Category (ArrowState s) where
    id                = ArrowState id
    arrow2 . arrow1   = ArrowState $ runArrowState arrow2 . runArrowState arrow1
  
  instance Arrow (ArrowState state) where
    arr               = ignoringState
    first  a          = ArrowState $ \(s,(aF,aS))
                                      -> second (,aS) $ runArrowState a (s,aF)
    second a          = ArrowState $ \(s,(aF,aS))
                                      -> second (aF,) $ runArrowState a (s,aS)
  
  instance ArrowChoice (ArrowState state) where
    left   a          = ArrowState $ \(s,e) -> case e of
                                   Left  l -> second Left  $ runArrowState a (s,l)
                                   Right r -> (s, Right r)
    right  a          = ArrowState $ \(s,e) -> case e of
                                   Left  l -> (s, Left l)
                                   Right r -> second Right $ runArrowState a (s,r)
  
  instance ArrowApply (ArrowState state) where
     app             = ArrowState $ \(s, (f,b)) -> runArrowState f (s,b)
  
  -- | Switches the type of the state temporarily.
  -- Drops the intermediate result state, behaving like a fallible
  -- identity arrow, save for side effects in the state.
  withSubStateF  :: ArrowState s  x (Either f s')
                 -> ArrowState s' s (Either f s )
                 -> ArrowState s  x (Either f x )
  withSubStateF  unlift a = keepingTheValue (withSubStateF' unlift a)
                            >>^ spreadChoice
                            >>^ fmap fst
  
  -- | Switches the type of the state temporarily.
  -- Returns the resulting sub-state.
  withSubStateF' :: ArrowState s  x (Either f s')
                 -> ArrowState s' s (Either f s )
                 -> ArrowState s  x (Either f s')
  withSubStateF' unlift a = ArrowState go
    where go p@(s,_) = tryRunning unlift
                                  ( tryRunning a (second Right) )
                                  p
            where tryRunning a' b v = case runArrowState a' v of
                                        (_ , Left  f) -> (s, Left f)
                                        (x , Right y) -> b (y,x)
  
  -- | Fold a state arrow through something 'Foldable'. Collect the results
  -- in a 'Monoid'.
  -- Intermediate form of a fold between one with "only" a 'Monoid'
  -- and one with any function.
  foldS :: (Foldable f, Monoid m) => ArrowState s x m -> ArrowState s (f x) m
  foldS a = ArrowState $ \(s,f) -> foldr a' (s,mempty) f
    where a' x (s',m) = second (mappend m)  $ runArrowState a (s',x)
  
  -- | Fold a state arrow through something 'Foldable'. Collect the results in a
  -- 'MonadPlus'.
  iterateS :: (Foldable f, MonadPlus m)
           => ArrowState s    x     y
           -> ArrowState s (f x) (m y)
  iterateS a = ArrowState $ \(s,f) -> foldr a' (s,mzero) f
    where a' x (s',m) = second (mplus m.return) $ runArrowState a (s',x)
  
  -- | Fold a state arrow through something 'Foldable'. Collect the results in a
  -- 'MonadPlus'.
  iterateSL :: (Foldable f, MonadPlus m)
            => ArrowState s    x     y
            -> ArrowState s (f x) (m y)
  iterateSL a = ArrowState $ \(s,f) -> foldl' a' (s,mzero) f
    where a' (s',m) x = second (mplus m.return) $ runArrowState a (s',x)
  
  
  -- | Fold a fallible state arrow through something 'Foldable'.
  -- Collect the results in a 'MonadPlus'.
  -- If the iteration fails, the state will be reset to the initial one.
  iterateS' :: (Foldable f, MonadPlus m)
            => ArrowState s    x  (Either e    y )
            -> ArrowState s (f x) (Either e (m y))
  iterateS' a = ArrowState $ \(s,f) -> foldr (a' s) (s,Right mzero) f
    where a' s x (s',Right m) = case runArrowState a (s',x) of
                                  (s'',Right m') -> (s'',Right $ mplus m $ return m')
                                  (_  ,Left  e ) -> (s  ,Left  e )
          a' _ _   e          = e
  