content: {-# LANGUAGE Trustworthy #-}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE RankNTypes #-}
  {-# LANGUAGE MagicHash #-}
  {-# LANGUAGE DeriveFunctor #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  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 GHC.Unicode ( isSpace )
  import GHC.List ( replicate, null )
  import GHC.Base hiding ( many )
  
  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 [(a,String)] -- invariant: list is non-empty!
    deriving Functor
  
  -- Monad, MonadPlus
  
  instance Applicative P where
    pure  = return
    (<*>) = ap
  
  instance MonadPlus P where
    mzero = empty
    mplus = (<|>)
  
  instance Monad P 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 <|> (p >>= k)
    (Final r)    >>= k = final [ys' | (x,s) <- r, ys' <- run (k x) s]
  
    fail _ = Fail
  
  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    <|> Look f     = Look (\s -> Final (r ++ run (f s) s))
    Final r    <|> p          = Look (\s -> Final (r ++ run p s))
    Look f     <|> Final r    = Look (\s -> Final (run (f s) s ++ r))
    p          <|> Final r    = Look (\s -> Final (run p s ++ 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)
  
  -- Functor, Monad, MonadPlus
  
  instance Functor ReadP where
    fmap h (R f) = R (\k -> f (k . h))
  
  instance Applicative ReadP where
      pure = return
      (<*>) = ap
  
  instance Monad ReadP 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 Alternative ReadP where
      empty = mzero
      (<|>) = mplus
  
  instance MonadPlus ReadP where
    mzero = pfail
    mplus = (+++)
  
  -- ---------------------------------------------------------------------------
  -- Operations over P
  
  final :: [(a,String)] -> P a
  -- Maintains invariant for Final constructor
  final [] = Fail
  final r  = Final r
  
  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)    _     = r
  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 (\_ -> Fail)
  
  (+++) :: ReadP a -> ReadP a -> ReadP a
  -- ^ Symmetric choice.
  R f1 +++ R f2 = R (\k -> f1 k <|> f2 k)
  
  (<++) :: 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 (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 (\s -> gath l (f s))
    gath l (Result k p) = k (l []) <|> gath l p
    gath _ (Final _)    = error "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
           ; if null s then return ()
                       else pfail }
  
  string :: String -> ReadP 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 String
  -- ^ Parses the first zero or more characters satisfying the predicate.
  --   Always succeds, 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 _            = do return ""
  
  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 _                 = do 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 = sequence (replicate 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 = (p >> return ()) +++ 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 = many p >> return ()
  
  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']))
  
  -- ---------------------------------------------------------------------------
  -- QuickCheck properties that hold for the combinators
  
  {- $properties
  The following are QuickCheck specifications of what the combinators do.
  These can be seen as formal specifications of the behavior of the
  combinators.
  
  We use bags to give semantics to the combinators.
  
  >  type Bag a = [a]
  
  Equality on bags does not care about the order of elements.
  
  >  (=~) :: Ord a => Bag a -> Bag a -> Bool
  >  xs =~ ys = sort xs == sort ys
  
  A special equality operator to avoid unresolved overloading
  when testing the properties.
  
  >  (=~.) :: Bag (Int,String) -> Bag (Int,String) -> Bool
  >  (=~.) = (=~)
  
  Here follow the properties:
  
  >  prop_Get_Nil =
  >    readP_to_S get [] =~ []
  >
  >  prop_Get_Cons c s =
  >    readP_to_S get (c:s) =~ [(c,s)]
  >
  >  prop_Look s =
  >    readP_to_S look s =~ [(s,s)]
  >
  >  prop_Fail s =
  >    readP_to_S pfail s =~. []
  >
  >  prop_Return x s =
  >    readP_to_S (return x) s =~. [(x,s)]
  >
  >  prop_Bind p k s =
  >    readP_to_S (p >>= k) s =~.
  >      [ ys''
  >      | (x,s') <- readP_to_S p s
  >      , ys''   <- readP_to_S (k (x::Int)) s'
  >      ]
  >
  >  prop_Plus p q s =
  >    readP_to_S (p +++ q) s =~.
  >      (readP_to_S p s ++ readP_to_S q s)
  >
  >  prop_LeftPlus p q s =
  >    readP_to_S (p <++ q) s =~.
  >      (readP_to_S p s +<+ readP_to_S q s)
  >   where
  >    [] +<+ ys = ys
  >    xs +<+ _  = xs
  >
  >  prop_Gather s =
  >    forAll readPWithoutReadS $ \p ->
  >      readP_to_S (gather p) s =~
  >        [ ((pre,x::Int),s')
  >        | (x,s') <- readP_to_S p s
  >        , let pre = take (length s - length s') s
  >        ]
  >
  >  prop_String_Yes this s =
  >    readP_to_S (string this) (this ++ s) =~
  >      [(this,s)]
  >
  >  prop_String_Maybe this s =
  >    readP_to_S (string this) s =~
  >      [(this, drop (length this) s) | this `isPrefixOf` s]
  >
  >  prop_Munch p s =
  >    readP_to_S (munch p) s =~
  >      [(takeWhile p s, dropWhile p s)]
  >
  >  prop_Munch1 p s =
  >    readP_to_S (munch1 p) s =~
  >      [(res,s') | let (res,s') = (takeWhile p s, dropWhile p s), not (null res)]
  >
  >  prop_Choice ps s =
  >    readP_to_S (choice ps) s =~.
  >      readP_to_S (foldr (+++) pfail ps) s
  >
  >  prop_ReadS r s =
  >    readP_to_S (readS_to_P r) s =~. r s
  -}
  
  