content: -----------------------------------------------------------------------------
  -- |
  -- Module      :  GHC.Internal.Text.ParserCombinators.ReadP
  -- Copyright   :  (c) The University of Glasgow 2002
  -- License     :  BSD-style (see the file libraries/base/LICENSE)
  --
  -- Maintainer  :  libraries@haskell.org
  -- Stability   :  provisional
  -- Portability :  non-portable (local universal quantification)
  --
  -- 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\".
  
  -----------------------------------------------------------------------------
  
  module Text.ParserCombinators.ReadP
    (
    -- * The 'ReadP' type
    ReadP,
  
    -- * Primitive operations
    get,
    look,
    (+++),
    (<++),
    gather,
  
    -- * Other operations
    pfail,
    eof,
    satisfy,
    char,
    string,
    munch,
    munch1,
    skipSpaces,
    choice,
    count,
    between,
    option,
    optional,
    many,
    many1,
    skipMany,
    skipMany1,
    sepBy,
    sepBy1,
    endBy,
    endBy1,
    chainr,
    chainl,
    chainl1,
    chainr1,
    manyTill,
  
    -- * Running a parser
    ReadS,
    readP_to_S,
    readS_to_P,
  
    -- * Properties
    -- $properties
    )
   where
  import qualified Prelude()
  import Primitives
  import Control.Applicative hiding (optional, many)
  import Control.Error
  import Control.Monad
  import Control.Monad.Fail
  import Data.Bool
  import Data.Char
  import Data.Eq
  import Data.Function
  import Data.Functor
  import Data.Int.Int
  import Data.Num
  import Data.List
  import Data.List.NonEmpty
  import Data.Monoid.Internal
  import {-# SOURCE #-} Data.Typeable
  
  default (Int)
  
  infixr 5 +++, <++
  
  ------------------------------------------------------------------------
  -- ReadS
  
  -- | A parser for a type @a@, represented as a function that takes a
  -- 'String' and returns a list of possible parses as @(a,'String')@ pairs.
  --
  -- Note that this kind of backtracking parser is very inefficient;
  -- reading a large structure may be quite slow (cf 'ReadP').
  type ReadS a = String -> [(a,String)]
  
  -- ---------------------------------------------------------------------------
  -- The P type
  -- is representation type -- should be kept abstract
  
  data P a
    = Get (Char -> P a)
    | Look (String -> P a)
    | Fail
    | Result a (P a)
    | Final (NonEmpty (a,String))
  --  deriving Functor -- ^ @since base-4.8.0.0
  
  instance Functor P where
    fmap f (Get g) = Get (fmap f . g)
    fmap f (Look g) = Look (fmap f . g)
    fmap _ Fail = Fail
    fmap f (Result a pa) = Result (f a) (fmap f pa)
    fmap f (Final xs) = Final (fmap (\ (a, s) -> (f a, s)) xs)
  
  -- Monad, MonadPlus
  
  -- | @since base-4.5.0.0
  instance Applicative P where
    pure x = Result x Fail
    (<*>) = ap
  
  -- | @since base-2.01
  instance MonadPlus P
  
  -- | @since base-2.01
  instance Monad P where
    (Get f)         >>= k = Get (f >=> k)
    (Look f)        >>= k = Look (f >=> k)
    Fail            >>= _ = Fail
    (Result x p)    >>= k = k x <|> (p >>= k)
    (Final (r:|rs)) >>= k = final [ys' | (x,s) <- r:rs, ys' <- run (k x) s]
  
  -- | @since base-4.9.0.0
  instance MonadFail P where
    fail _ = Fail
  
  -- | @since base-4.5.0.0
  instance Alternative P where
    empty = Fail
  
    -- most common case: two gets are combined
    Get f1     <|> Get f2     = Get (\c -> f1 c <|> f2 c)
  
    -- results are delivered as soon as possible
    Result x p <|> q          = Result x (p <|> q)
    p          <|> Result x q = Result x (p <|> q)
  
    -- fail disappears
    Fail       <|> p          = p
    p          <|> 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       <|> Final t = Final (r <> t)
    Final (r:|rs) <|> Look f  = Look (\s -> Final (r:|(rs ++ run (f s) s)))
    Final (r:|rs) <|> p       = Look (\s -> Final (r:|(rs ++ run p s)))
    Look f        <|> Final r = Look (\s -> Final (case run (f s) s of
                                  []     -> r
                                  (x:xs) -> (x:|xs) <> r))
    p             <|> Final r = Look (\s -> Final (case run p s of
                                  []     -> r
                                  (x:xs) -> (x:|xs) <> r))
  
    -- two looks are combined (=optimization)
    -- look + sthg else floats upwards
    Look f     <|> Look g     = Look (\s -> f s <|> g s)
    Look f     <|> p          = Look (\s -> f s <|> p)
    p          <|> Look f     = Look (\s -> p <|> f s)
  
  -- ---------------------------------------------------------------------------
  -- The ReadP type
  
  newtype ReadP a = R (forall b . (a -> P b) -> P b)
  
  -- | @since base-2.01
  instance Functor ReadP where
    fmap h (R f) = R (\k -> f (k . h))
  
  -- | @since base-4.6.0.0
  instance Applicative ReadP where
      pure x = R (\k -> k x)
      (<*>) = ap
      -- liftA2 = liftM2
  
  -- | @since base-2.01
  instance Monad ReadP where
    R m >>= f = R (\k -> m (\a -> let R m' = f a in m' k))
  
  -- | @since base-4.9.0.0
  instance MonadFail ReadP where
    fail _    = R (const Fail)
  
  -- | @since base-4.6.0.0
  instance Alternative ReadP where
    empty = pfail
    (<|>) = (+++)
  
  -- | @since base-2.01
  instance MonadPlus ReadP
  
  -- ---------------------------------------------------------------------------
  -- Operations over P
  
  final :: [(a,String)] -> P a
  final []     = Fail
  final (r:rs) = Final (r:|rs)
  
  run :: P a -> ReadS a
  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:|rs)) _     = r:rs
  run _               _     = []
  
  -- ---------------------------------------------------------------------------
  -- Operations over ReadP
  
  get :: ReadP Char
  -- ^ Consumes and returns the next character.
  --   Fails if there is no input left.
  get = R Get
  
  look :: ReadP String
  -- ^ Look-ahead: returns the part of the input that is left, without
  --   consuming it.
  look = R Look
  
  pfail :: ReadP a
  -- ^ Always fails.
  pfail = R (const Fail)
  
  (+++) :: ReadP a -> ReadP a -> ReadP a
  -- ^ Symmetric choice.
  R f1 +++ R f2 = R (\k -> f1 k <|> f2 k)
  
  (<++) :: forall a . ReadP a -> ReadP a -> ReadP a
  -- ^ Local, exclusive, left-biased choice: If left parser
  --   locally produces any result at all, then right parser is
  --   not used.
  (<++) (R f0) q =
    do s <- look
       probe (f0 return) s 0
   where
    probe :: P a -> [Char] -> Int -> ReadP a
    probe (Get f)        (c:s) n = probe (f c) s (n + 1)
    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)
  
  gather :: ReadP a -> ReadP (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 :: (String -> String) -> P (String -> P b) -> P b
    gath l (Get f)      = Get (\c -> gath (l.(c:)) (f c))
    gath _ Fail         = Fail
    gath l (Look f)     = Look (gath l . f)
    gath l (Result k p) = k (l []) <|> gath l p
    gath _ (Final _)    = errorWithoutStackTrace "do not use readS_to_P in gather!"
  
  -- ---------------------------------------------------------------------------
  -- Derived operations
  
  satisfy :: (Char -> Bool) -> ReadP 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 Char
  -- ^ Parses and returns the specified character.
  char c = satisfy (c ==)
  
  eof :: ReadP ()
  -- ^ Succeeds iff we are at the end of input
  eof = do { s <- look
           ; unless (null s) pfail }
  
  string :: String -> ReadP String
  -- ^ Parses and returns the specified string.
  string this = do s <- look; scan this s
   where
    scan []     _               = return this
    scan (x:xs) (y:ys) | x == y = do _ <- get; scan xs ys
    scan _      _               = pfail
  
  munch :: (Char -> Bool) -> ReadP String
  -- ^ Parses the first zero or more characters satisfying the predicate.
  --   Always succeeds, exactly once having consumed all the characters
  --   Hence NOT the same as (many (satisfy p))
  munch p =
    do s <- look
       scan s
   where
    scan (c:cs) | p c = do _ <- get; s <- scan cs; return (c:s)
    scan _            = return (""::String)
  
  munch1 :: (Char -> Bool) -> ReadP String
  -- ^ Parses the first one or more characters satisfying the predicate.
  --   Fails if none, else succeeds exactly once having consumed all the characters
  --   Hence NOT the same as (many1 (satisfy p))
  munch1 p =
    do c <- get
       if p c then do s <- munch p; return (c:s)
              else pfail
  
  choice :: [ReadP a] -> ReadP a
  -- ^ Combines all parsers in the specified list.
  choice []     = pfail
  choice [p]    = p
  choice (p:ps) = p +++ choice ps
  
  skipSpaces :: ReadP ()
  -- ^ Skips all whitespace.
  skipSpaces =
    do s <- look
       skip s
   where
    skip (c:s) | isSpace c = do _ <- get; skip s
    skip _                 = return ()
  
  count :: Int -> ReadP a -> ReadP [a]
  -- ^ @count n p@ parses @n@ occurrences of @p@ in sequence. A list of
  --   results is returned.
  count n p = replicateM n p
  
  between :: ReadP open -> ReadP close -> ReadP a -> ReadP 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 a -> ReadP a
  -- ^ @option x p@ will either parse @p@ or return @x@ without consuming
  --   any input.
  option x p = p +++ return x
  
  optional :: ReadP a -> ReadP ()
  -- ^ @optional p@ optionally parses @p@ and always returns @()@.
  optional p = void p +++ return ()
  
  many :: ReadP a -> ReadP [a]
  -- ^ Parses zero or more occurrences of the given parser.
  many p = return [] +++ many1 p
  
  many1 :: ReadP a -> ReadP [a]
  -- ^ Parses one or more occurrences of the given parser.
  many1 p = liftM2 (:) p (many p)
  
  skipMany :: ReadP a -> ReadP ()
  -- ^ Like 'many', but discards the result.
  skipMany p = void (many p)
  
  skipMany1 :: ReadP a -> ReadP ()
  -- ^ Like 'many1', but discards the result.
  skipMany1 p = p >> skipMany p
  
  sepBy :: ReadP a -> ReadP sep -> ReadP [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 a -> ReadP sep -> ReadP [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 a -> ReadP sep -> ReadP [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 a -> ReadP sep -> ReadP [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 a -> ReadP (a -> a -> a) -> a -> ReadP 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 a -> ReadP (a -> a -> a) -> a -> ReadP 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 a -> ReadP (a -> a -> a) -> ReadP 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 a -> ReadP (a -> a -> a) -> ReadP 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 a -> ReadP end -> ReadP [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 -> 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 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']))
  