    {-# LANGUAGE GeneralizedNewtypeDeriving #-}

    {-
    Copyright (C) 2015 Martin Linnemann <theCodingMarlin@googlemail.com>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    -}

    {- |
       Module      : Text.Pandoc.Readers.Odt.Generic.Fallible
       Copyright   : Copyright (C) 2015 Martin Linnemann
       License     : GNU GPL, version 2 or above

       Maintainer  : Martin Linnemann <theCodingMarlin@googlemail.com>
       Stability   : alpha
       Portability : portable

    Data types and utilities representing failure. Most of it is based on the
    "Either" type in its usual configuration (left represents failure).

    In most cases, the failure type is implied or required to be a "Monoid".

    The choice of "Either" instead of a custom type makes it easier to write
    compatible instances of "ArrowChoice".
    -}

    -- We export everything
    module Text.Pandoc.Readers.Odt.Generic.Fallible where

    import           Control.Applicative
    import           Control.Monad

    import qualified Data.Foldable       as F
    import           Data.Monoid ((<>))

    -- | Default for now. Will probably become a class at some point.
    type Failure = ()

    type Fallible a = Either Failure a

    -- | False -> Left (), True -> Right ()
    boolToEither :: Bool -> Fallible ()
    boolToEither False = Left  ()
    boolToEither True  = Right ()

    -- | False -> Left (), True -> Right ()
    boolToChoice :: Bool -> Fallible ()
    boolToChoice False = Left  ()
    boolToChoice True  = Right ()

    --
    maybeToEither :: Maybe a -> Fallible a
    maybeToEither (Just a) = Right a
    maybeToEither Nothing  = Left  ()

    --
    eitherToMaybe :: Either _l a -> Maybe a
    eitherToMaybe (Left  _) = Nothing
    eitherToMaybe (Right a) = Just a

    -- | > untagEither === either id id
    untagEither :: Either a a -> a
    untagEither (Left  a) = a
    untagEither (Right a) = a

    -- | > fromLeft f === either f id
    fromLeft :: (a -> b) -> Either a b -> b
    fromLeft f (Left  a) = f a
    fromLeft _ (Right b) = b

    -- | > fromRight f === either id f
    fromRight :: (a -> b) -> Either b a -> b
    fromRight _ (Left  b) = b
    fromRight f (Right a) = f a

    -- | > recover a === fromLeft (const a) === either (const a) id
    recover :: a -> Either _f a -> a
    recover a (Left  _) = a
    recover _ (Right a) = a

    -- | I would love to use 'fail'. Alas, 'Monad.fail'...
    failWith :: failure -> Either failure _x
    failWith f = Left f

    --
    failEmpty :: (Monoid failure) => Either failure _x
    failEmpty = failWith mempty

    --
    succeedWith :: a -> Either _x a
    succeedWith = Right

    --
    collapseEither :: Either failure (Either failure x)
                   -> Either failure x
    collapseEither (Left f         ) = Left f
    collapseEither (Right (Left  f)) = Left f
    collapseEither (Right (Right x)) = Right x

    -- | If either of the values represents an error, the result is a
    -- (possibly combined) error. If both values represent a success,
    -- both are returned.
    chooseMin :: (Monoid a) => Either a b -> Either a b' -> Either a (b,b')
    chooseMin = chooseMinWith (,)

    -- | If either of the values represents an error, the result is a
    -- (possibly combined) error. If both values represent a success,
    -- a combination is returned.
    chooseMinWith :: (Monoid a) => (b -> b' -> c)
                                -> Either a b
                                -> Either a b'
                                -> Either a c
    chooseMinWith (><) (Right a) (Right b) = Right $ a >< b
    chooseMinWith  _   (Left  a) (Left  b) = Left  $ a <> b
    chooseMinWith  _   (Left  a)     _     = Left  a
    chooseMinWith  _       _     (Left  b) = Left  b

    -- | If either of the values represents a  non-error, the result is a
    -- (possibly combined) non-error. If both values represent an error, an error
    -- is returned.
    chooseMax :: (Monoid a, Monoid b) => Either a b -> Either a b -> Either a b
    chooseMax = chooseMaxWith (<>)

    -- | If either of the values represents a non-error, the result is a
    -- (possibly combined) non-error. If both values represent an error, an error
    -- is returned.
    chooseMaxWith :: (Monoid a) => (b -> b -> b)
                                -> Either a b
                                -> Either a b
                                -> Either a b
    chooseMaxWith (><) (Right a) (Right b) = Right $ a >< b
    chooseMaxWith  _   (Left  a) (Left  b) = Left  $ a <> b
    chooseMaxWith  _   (Right a)     _     = Right a
    chooseMaxWith  _       _     (Right b) = Right b

    -- | Class of containers that can escalate contained 'Either's.
    -- The word "Vector" is meant in the sense of a disease transmitter.
    class ChoiceVector v where
      spreadChoice :: v (Either f a) -> Either f (v a)

    -- Let's do a few examples first

    instance ChoiceVector Maybe where
      spreadChoice (Just (Left  f))  = Left  f
      spreadChoice (Just (Right x))  = Right (Just x)
      spreadChoice Nothing           = Right Nothing

    instance ChoiceVector (Either l) where
      spreadChoice (Right (Left  f)) = Left  f
      spreadChoice (Right (Right x)) = Right (Right x)
      spreadChoice (Left   x       ) = Right (Left x)

    instance ChoiceVector ((,) a) where
      spreadChoice (_, Left  f) = Left  f
      spreadChoice (x, Right y) = Right (x,y)
      -- Wasn't there a newtype somewhere with the elements flipped?

    --
    -- More instances later, first some discussion.
    --
    -- I'll have to freshen up on type system details to see how (or if) to do
    -- something like
    --
    -- > instance (ChoiceVector a, ChoiceVector b) => ChoiceVector (a b) where
    -- >   :
    --
    -- But maybe it would be even better to use something like
    --
    -- > class ChoiceVector v v' f | v -> v' f where
    -- >   spreadChoice :: v -> Either f v'
    --
    -- That way, more places in @v@ could spread the cheer, e.g.:
    --
    -- As before:
    -- --                       ( a , Either f b)    (a , b)  f
    -- > instance ChoiceVector ((,) a (Either f b)) ((,) a b) f where
    -- >   spreadChoice (_, Left  f) = Left f
    -- >   spreadChoice (a, Right b) = Right (a,b)
    --
    -- But also:
    -- --                       ( Either f a , b)    (a , b)  f
    -- > instance ChoiceVector ((,) (Either f a) b) ((,) a b) f where
    -- >   spreadChoice (Right a,b) = Right (a,b)
    -- >   spreadChoice (Left  f,_) = Left f
    --
    -- And maybe even:
    -- --                        ( Either f a , Either f b)     (a , b)  f
    -- > instance ChoiceVector ((,) (Either f a) (Either f b)) ((,) a b) f where
    -- >   spreadChoice (Right a , Right b) = Right (a,b)
    -- >   spreadChoice (Left  f , _      ) = Left f
    -- >   spreadChoice ( _      , Left  f) = Left f
    --
    -- Of course that would lead to a lot of overlapping instances...
    -- But I can't think of a different way. A selector function might help,
    -- but not even a "Data.Traversable" is powerful enough for that.
    -- But maybe someone has already solved all this with a lens library.
    --
    -- Well, it's an interesting academic question. But for practical purposes,
    -- I have more than enough right now.

    instance ChoiceVector ((,,) a b) where
      spreadChoice (_,_, Left  f) = Left  f
      spreadChoice (a,b, Right x) = Right (a,b,x)

    instance ChoiceVector ((,,,) a b c) where
      spreadChoice (_,_,_, Left  f) = Left  f
      spreadChoice (a,b,c, Right x) = Right (a,b,c,x)

    instance ChoiceVector ((,,,,) a b c d) where
      spreadChoice (_,_,_,_, Left  f) = Left  f
      spreadChoice (a,b,c,d, Right x) = Right (a,b,c,d,x)

    instance ChoiceVector (Const a) where
      spreadChoice (Const c) = Right (Const c) -- need to repackage because of implicit types

    -- | Fails on the first error
    instance ChoiceVector [] where
      spreadChoice = sequence -- using the monad instance of Either.
      -- Could be generalized to "Data.Traversable" - but why play
      -- with UndecidableInstances unless this is really needed.

    -- | Wrapper for a list. While the normal list instance of 'ChoiceVector'
    -- fails whenever it can, this type will never fail.
    newtype SuccessList a = SuccessList { collectNonFailing :: [a] }
      deriving ( Eq, Ord, Show )

    instance ChoiceVector SuccessList  where
      spreadChoice = Right . SuccessList . (foldr unTagRight []) . collectNonFailing
        where unTagRight (Right x) = (x:)
              unTagRight _         = id

    -- | Like 'catMaybes', but for 'Either'.
    collectRights :: [Either _l r] -> [r]
    collectRights = collectNonFailing . untag . spreadChoice . SuccessList
      where untag = fromLeft (error "Unexpected Left")

    -- | A version of 'collectRights' generalized to other containers. The
    -- container must be both "reducible" and "buildable". Most general containers
    -- should fullfill these requirements, but there is no single typeclass
    -- (that I know of) for that.
    -- Therefore, they are split between 'Foldable' and 'MonadPlus'.
    -- (Note that 'Data.Traversable.Traversable' alone would not be enough, either.)
    collectRightsF :: (F.Foldable c, MonadPlus c) => c (Either _l r) ->  c r
    collectRightsF = F.foldr unTagRight mzero
      where unTagRight (Right x) = mplus $ return x
            unTagRight _         = id