content: {- |
     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
    ( Failure
    , Fallible
    , maybeToEither
    , eitherToMaybe
    , recover
    , failWith
    , failEmpty
    , succeedWith
    , collapseEither
    , chooseMax
    , chooseMaxWith
    , ChoiceVector(..)
    , SuccessList(..)
    ) where
  
  -- | Default for now. Will probably become a class at some point.
  type Failure = ()
  
  type Fallible a = Either Failure a
  
  
  --
  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
  
  -- | > recover 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 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 mappend
  
  -- | 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 `mappend` 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)
  
  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?
  
  -- | 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
  