    {-# LANGUAGE CPP #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Distribution.Compat.ReadP
    -- Copyright   :  (c) The University of Glasgow 2002
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Portability :  portable
    --
    -- This is a library of parser combinators, originally written by Koen Claessen.
    -- It parses all alternatives in parallel, so it never keeps hold of
    -- the beginning of the input string, a common source of space leaks with
    -- other parsers.  The '(+++)' choice combinator is genuinely commutative;
    -- it makes no difference which branch is \"shorter\".
    --
    -- See also Koen's paper /Parallel Parsing Processes/
    -- (<http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.19.9217>).
    --
    -- This version of ReadP has been locally hacked to make it H98, by
    -- Martin Sj&#xF6;gren <mailto:msjogren@gmail.com>
    --
    -- The unit tests have been moved to UnitTest.Distribution.Compat.ReadP, by
    -- Mark Lentczner <mailto:mark@glyphic.com>
    -----------------------------------------------------------------------------

    module Distribution.Compat.ReadP
      (
      -- * The 'ReadP' type
      ReadP,      -- :: * -> *; instance Functor, Monad, MonadPlus

      -- * Primitive operations
      get,        -- :: ReadP Char
      look,       -- :: ReadP String
      (+++),      -- :: ReadP a -> ReadP a -> ReadP a
      (<++),      -- :: ReadP a -> ReadP a -> ReadP a
      gather,     -- :: ReadP a -> ReadP (String, a)

      -- * Other operations
      pfail,      -- :: ReadP a
      satisfy,    -- :: (Char -> Bool) -> ReadP Char
      char,       -- :: Char -> ReadP Char
      string,     -- :: String -> ReadP String
      munch,      -- :: (Char -> Bool) -> ReadP String
      munch1,     -- :: (Char -> Bool) -> ReadP String
      skipSpaces, -- :: ReadP ()
      choice,     -- :: [ReadP a] -> ReadP a
      count,      -- :: Int -> ReadP a -> ReadP [a]
      between,    -- :: ReadP open -> ReadP close -> ReadP a -> ReadP a
      option,     -- :: a -> ReadP a -> ReadP a
      optional,   -- :: ReadP a -> ReadP ()
      many,       -- :: ReadP a -> ReadP [a]
      many1,      -- :: ReadP a -> ReadP [a]
      skipMany,   -- :: ReadP a -> ReadP ()
      skipMany1,  -- :: ReadP a -> ReadP ()
      sepBy,      -- :: ReadP a -> ReadP sep -> ReadP [a]
      sepBy1,     -- :: ReadP a -> ReadP sep -> ReadP [a]
      endBy,      -- :: ReadP a -> ReadP sep -> ReadP [a]
      endBy1,     -- :: ReadP a -> ReadP sep -> ReadP [a]
      chainr,     -- :: ReadP a -> ReadP (a -> a -> a) -> a -> ReadP a
      chainl,     -- :: ReadP a -> ReadP (a -> a -> a) -> a -> ReadP a
      chainl1,    -- :: ReadP a -> ReadP (a -> a -> a) -> ReadP a
      chainr1,    -- :: ReadP a -> ReadP (a -> a -> a) -> ReadP a
      manyTill,   -- :: ReadP a -> ReadP end -> ReadP [a]

      -- * Running a parser
      ReadS,      -- :: *; = String -> [(a,String)]
      readP_to_S, -- :: ReadP a -> ReadS a
      readS_to_P  -- :: ReadS a -> ReadP a
      )
     where

    import Control.Monad( MonadPlus(..), liftM, liftM2, ap )
    import Data.Char (isSpace)
    #if __GLASGOW_HASKELL__ < 710
    import Control.Applicative (Applicative(..))
    #endif
    import Control.Applicative (Alternative(empty, (<|>)))

    infixr 5 +++, <++

    -- ---------------------------------------------------------------------------
    -- The P type
    -- is representation type -- should be kept abstract

    data P s a
      = Get (s -> P s a)
      | Look ([s] -> P s a)
      | Fail
      | Result a (P s a)
      | Final [(a,[s])] -- invariant: list is non-empty!

    -- Monad, MonadPlus

    instance Functor (P s) where
      fmap = liftM

    instance Applicative (P s) where
      pure = return
      (<*>) = ap

    instance Monad (P s) where
      return x = Result x Fail

      (Get f)      >>= k = Get (\c -> f c >>= k)
      (Look f)     >>= k = Look (\s -> f s >>= k)
      Fail         >>= _ = Fail
      (Result x p) >>= k = k x `mplus` (p >>= k)
      (Final r)    >>= k = final [ys' | (x,s) <- r, ys' <- run (k x) s]

      fail _ = Fail

    instance Alternative (P s) where
          empty = mzero
          (<|>) = mplus

    instance MonadPlus (P s) where
      mzero = Fail

      -- most common case: two gets are combined
      Get f1     `mplus` Get f2     = Get (\c -> f1 c `mplus` f2 c)

      -- results are delivered as soon as possible
      Result x p `mplus` q          = Result x (p `mplus` q)
      p          `mplus` Result x q = Result x (p `mplus` q)

      -- fail disappears
      Fail       `mplus` p          = p
      p          `mplus` Fail       = p

      -- two finals are combined
      -- final + look becomes one look and one final (=optimization)
      -- final + sthg else becomes one look and one final
      Final r    `mplus` Final t    = Final (r ++ t)
      Final r    `mplus` Look f     = Look (\s -> Final (r ++ run (f s) s))
      Final r    `mplus` p          = Look (\s -> Final (r ++ run p s))
      Look f     `mplus` Final r    = Look (\s -> Final (run (f s) s ++ r))
      p          `mplus` Final r    = Look (\s -> Final (run p s ++ r))

      -- two looks are combined (=optimization)
      -- look + sthg else floats upwards
      Look f     `mplus` Look g     = Look (\s -> f s `mplus` g s)
      Look f     `mplus` p          = Look (\s -> f s `mplus` p)
      p          `mplus` Look f     = Look (\s -> p `mplus` f s)

    -- ---------------------------------------------------------------------------
    -- The ReadP type

    newtype Parser r s a = R ((a -> P s r) -> P s r)
    type ReadP r a = Parser r Char a

    -- Functor, Monad, MonadPlus

    instance Functor (Parser r s) where
      fmap h (R f) = R (\k -> f (k . h))

    instance Applicative (Parser r s) where
      pure = return
      (<*>) = ap

    instance Monad (Parser r s) where
      return x  = R (\k -> k x)
      fail _    = R (\_ -> Fail)
      R m >>= f = R (\k -> m (\a -> let R m' = f a in m' k))

    --instance MonadPlus (Parser r s) where
    --  mzero = pfail
    --  mplus = (+++)

    -- ---------------------------------------------------------------------------
    -- Operations over P

    final :: [(a,[s])] -> P s a
    -- Maintains invariant for Final constructor
    final [] = Fail
    final r  = Final r

    run :: P c a -> ([c] -> [(a, [c])])
    run (Get f)      (c:s) = run (f c) s
    run (Look f)     s     = run (f s) s
    run (Result x p) s     = (x,s) : run p s
    run (Final r)    _     = r
    run _            _     = []

    -- ---------------------------------------------------------------------------
    -- Operations over ReadP

    get :: ReadP r Char
    -- ^ Consumes and returns the next character.
    --   Fails if there is no input left.
    get = R Get

    look :: ReadP r String
    -- ^ Look-ahead: returns the part of the input that is left, without
    --   consuming it.
    look = R Look

    pfail :: ReadP r a
    -- ^ Always fails.
    pfail = R (\_ -> Fail)

    (+++) :: ReadP r a -> ReadP r a -> ReadP r a
    -- ^ Symmetric choice.
    R f1 +++ R f2 = R (\k -> f1 k `mplus` f2 k)

    (<++) :: ReadP a a -> ReadP r a -> ReadP r a
    -- ^ Local, exclusive, left-biased choice: If left parser
    --   locally produces any result at all, then right parser is
    --   not used.
    R f <++ q =
      do s <- look
         probe (f return) s 0
     where
      probe (Get f')       (c:s) n = probe (f' c) s (n+1 :: Int)
      probe (Look f')      s     n = probe (f' s) s n
      probe p@(Result _ _) _     n = discard n >> R (p >>=)
      probe (Final r)      _     _ = R (Final r >>=)
      probe _              _     _ = q

      discard 0 = return ()
      discard n  = get >> discard (n-1 :: Int)

    gather :: ReadP (String -> P Char r) a -> ReadP r (String, a)
    -- ^ Transforms a parser into one that does the same, but
    --   in addition returns the exact characters read.
    --   IMPORTANT NOTE: 'gather' gives a runtime error if its first argument
    --   is built using any occurrences of readS_to_P.
    gather (R m) =
      R (\k -> gath id (m (\a -> return (\s -> k (s,a)))))
     where
      gath l (Get f)      = Get (\c -> gath (l.(c:)) (f c))
      gath _ Fail         = Fail
      gath l (Look f)     = Look (\s -> gath l (f s))
      gath l (Result k p) = k (l []) `mplus` gath l p
      gath _ (Final _)    = error "do not use readS_to_P in gather!"

    -- ---------------------------------------------------------------------------
    -- Derived operations

    satisfy :: (Char -> Bool) -> ReadP r Char
    -- ^ Consumes and returns the next character, if it satisfies the
    --   specified predicate.
    satisfy p = do c <- get; if p c then return c else pfail

    char :: Char -> ReadP r Char
    -- ^ Parses and returns the specified character.
    char c = satisfy (c ==)

    string :: String -> ReadP r String
    -- ^ Parses and returns the specified string.
    string this = do s <- look; scan this s
     where
      scan []     _               = do return this
      scan (x:xs) (y:ys) | x == y = do get >> scan xs ys
      scan _      _               = do pfail

    munch :: (Char -> Bool) -> ReadP r String
    -- ^ Parses the first zero or more characters satisfying the predicate.
    munch p =
      do s <- look
         scan s
     where
      scan (c:cs) | p c = do _ <- get; s <- scan cs; return (c:s)
      scan _            = do return ""

    munch1 :: (Char -> Bool) -> ReadP r String
    -- ^ Parses the first one or more characters satisfying the predicate.
    munch1 p =
      do c <- get
         if p c then do s <- munch p; return (c:s)
                else pfail

    choice :: [ReadP r a] -> ReadP r a
    -- ^ Combines all parsers in the specified list.
    choice []     = pfail
    choice [p]    = p
    choice (p:ps) = p +++ choice ps

    skipSpaces :: ReadP r ()
    -- ^ Skips all whitespace.
    skipSpaces =
      do s <- look
         skip s
     where
      skip (c:s) | isSpace c = do _ <- get; skip s
      skip _                 = do return ()

    count :: Int -> ReadP r a -> ReadP r [a]
    -- ^ @ count n p @ parses @n@ occurrences of @p@ in sequence. A list of
    --   results is returned.
    count n p = sequence (replicate n p)

    between :: ReadP r open -> ReadP r close -> ReadP r a -> ReadP r a
    -- ^ @ between open close p @ parses @open@, followed by @p@ and finally
    --   @close@. Only the value of @p@ is returned.
    between open close p = do _ <- open
                              x <- p
                              _ <- close
                              return x

    option :: a -> ReadP r a -> ReadP r a
    -- ^ @option x p@ will either parse @p@ or return @x@ without consuming
    --   any input.
    option x p = p +++ return x

    optional :: ReadP r a -> ReadP r ()
    -- ^ @optional p@ optionally parses @p@ and always returns @()@.
    optional p = (p >> return ()) +++ return ()

    many :: ReadP r a -> ReadP r [a]
    -- ^ Parses zero or more occurrences of the given parser.
    many p = return [] +++ many1 p

    many1 :: ReadP r a -> ReadP r [a]
    -- ^ Parses one or more occurrences of the given parser.
    many1 p = liftM2 (:) p (many p)

    skipMany :: ReadP r a -> ReadP r ()
    -- ^ Like 'many', but discards the result.
    skipMany p = many p >> return ()

    skipMany1 :: ReadP r a -> ReadP r ()
    -- ^ Like 'many1', but discards the result.
    skipMany1 p = p >> skipMany p

    sepBy :: ReadP r a -> ReadP r sep -> ReadP r [a]
    -- ^ @sepBy p sep@ parses zero or more occurrences of @p@, separated by @sep@.
    --   Returns a list of values returned by @p@.
    sepBy p sep = sepBy1 p sep +++ return []

    sepBy1 :: ReadP r a -> ReadP r sep -> ReadP r [a]
    -- ^ @sepBy1 p sep@ parses one or more occurrences of @p@, separated by @sep@.
    --   Returns a list of values returned by @p@.
    sepBy1 p sep = liftM2 (:) p (many (sep >> p))

    endBy :: ReadP r a -> ReadP r sep -> ReadP r [a]
    -- ^ @endBy p sep@ parses zero or more occurrences of @p@, separated and ended
    --   by @sep@.
    endBy p sep = many (do x <- p ; _ <- sep ; return x)

    endBy1 :: ReadP r a -> ReadP r sep -> ReadP r [a]
    -- ^ @endBy p sep@ parses one or more occurrences of @p@, separated and ended
    --   by @sep@.
    endBy1 p sep = many1 (do x <- p ; _ <- sep ; return x)

    chainr :: ReadP r a -> ReadP r (a -> a -> a) -> a -> ReadP r a
    -- ^ @chainr p op x@ parses zero or more occurrences of @p@, separated by @op@.
    --   Returns a value produced by a /right/ associative application of all
    --   functions returned by @op@. If there are no occurrences of @p@, @x@ is
    --   returned.
    chainr p op x = chainr1 p op +++ return x

    chainl :: ReadP r a -> ReadP r (a -> a -> a) -> a -> ReadP r a
    -- ^ @chainl p op x@ parses zero or more occurrences of @p@, separated by @op@.
    --   Returns a value produced by a /left/ associative application of all
    --   functions returned by @op@. If there are no occurrences of @p@, @x@ is
    --   returned.
    chainl p op x = chainl1 p op +++ return x

    chainr1 :: ReadP r a -> ReadP r (a -> a -> a) -> ReadP r a
    -- ^ Like 'chainr', but parses one or more occurrences of @p@.
    chainr1 p op = scan
      where scan   = p >>= rest
            rest x = do f <- op
                        y <- scan
                        return (f x y)
                     +++ return x

    chainl1 :: ReadP r a -> ReadP r (a -> a -> a) -> ReadP r a
    -- ^ Like 'chainl', but parses one or more occurrences of @p@.
    chainl1 p op = p >>= rest
      where rest x = do f <- op
                        y <- p
                        rest (f x y)
                     +++ return x

    manyTill :: ReadP r a -> ReadP [a] end -> ReadP r [a]
    -- ^ @manyTill p end@ parses zero or more occurrences of @p@, until @end@
    --   succeeds. Returns a list of values returned by @p@.
    manyTill p end = scan
      where scan = (end >> return []) <++ (liftM2 (:) p scan)

    -- ---------------------------------------------------------------------------
    -- Converting between ReadP and Read

    readP_to_S :: ReadP a a -> ReadS a
    -- ^ Converts a parser into a Haskell ReadS-style function.
    --   This is the main way in which you can \"run\" a 'ReadP' parser:
    --   the expanded type is
    -- @ readP_to_S :: ReadP a -> String -> [(a,String)] @
    readP_to_S (R f) = run (f return)

    readS_to_P :: ReadS a -> ReadP r a
    -- ^ Converts a Haskell ReadS-style function into a parser.
    --   Warning: This introduces local backtracking in the resulting
    --   parser, and therefore a possible inefficiency.
    readS_to_P r =
      R (\k -> Look (\s -> final [bs'' | (a,s') <- r s, bs'' <- run (k a) s']))