    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Text.Parsec.Combinator
    -- Copyright   :  (c) Daan Leijen 1999-2001, (c) Paolo Martini 2007
    -- License     :  BSD-style (see the LICENSE file)
    --
    -- Maintainer  :  derek.a.elkins@gmail.com
    -- Stability   :  provisional
    -- Portability :  portable
    --
    -- Commonly used generic combinators
    --
    -----------------------------------------------------------------------------

    module Text.Parsec.Combinator
        ( choice
        , count
        , between
        , option, optionMaybe, optional
        , skipMany1
        , many1
        , sepBy, sepBy1
        , endBy, endBy1
        , sepEndBy, sepEndBy1
        , chainl, chainl1
        , chainr, chainr1
        , eof, notFollowedBy
        -- tricky combinators
        , manyTill, lookAhead, anyToken
        ) where

    import Control.Monad
    import Text.Parsec.Prim

    -- | @choice ps@ tries to apply the parsers in the list @ps@ in order,
    -- until one of them succeeds. Returns the value of the succeeding
    -- parser.

    choice :: (Stream s m t) => [ParsecT s u m a] -> ParsecT s u m a
    choice ps           = foldr (<|>) mzero ps

    -- | @option x p@ tries to apply parser @p@. If @p@ fails without
    -- consuming input, it returns the value @x@, otherwise the value
    -- returned by @p@.
    --
    -- >  priority  = option 0 (do{ d <- digit
    -- >                          ; return (digitToInt d)
    -- >                          })

    option :: (Stream s m t) => a -> ParsecT s u m a -> ParsecT s u m a
    option x p          = p <|> return x

    -- | @optionMaybe p@ tries to apply parser @p@.  If @p@ fails without
    -- consuming input, it return 'Nothing', otherwise it returns
    -- 'Just' the value returned by @p@.

    optionMaybe :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m (Maybe a)
    optionMaybe p       = option Nothing (liftM Just p)

    -- | @optional p@ tries to apply parser @p@.  It will parse @p@ or nothing.
    -- It only fails if @p@ fails after consuming input. It discards the result
    -- of @p@.

    optional :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m ()
    optional p          = do{ p; return ()} <|> return ()

    -- | @between open close p@ parses @open@, followed by @p@ and @close@.
    -- Returns the value returned by @p@.
    --
    -- >  braces  = between (symbol "{") (symbol "}")

    between :: (Stream s m t) => ParsecT s u m open -> ParsecT s u m close
                -> ParsecT s u m a -> ParsecT s u m a
    between open close p
                        = do{ open; x <- p; close; return x }

    -- | @skipMany1 p@ applies the parser @p@ /one/ or more times, skipping
    -- its result.

    skipMany1 :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m ()
    skipMany1 p         = do{ p; skipMany p }
    {-
    skipMany p          = scan
                        where
                          scan  = do{ p; scan } <|> return ()
    -}

    -- | @many1 p@ applies the parser @p@ /one/ or more times. Returns a
    -- list of the returned values of @p@.
    --
    -- >  word  = many1 letter

    many1 :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m [a]
    many1 p             = do{ x <- p; xs <- many p; return (x:xs) }
    {-
    many p              = scan id
                        where
                          scan f    = do{ x <- p
                                        ; scan (\tail -> f (x:tail))
                                        }
                                    <|> return (f [])
    -}

    -- | @sepBy p sep@ parses /zero/ or more occurrences of @p@, separated
    -- by @sep@. Returns a list of values returned by @p@.
    --
    -- >  commaSep p  = p `sepBy` (symbol ",")

    sepBy :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]
    sepBy p sep         = sepBy1 p sep <|> return []

    -- | @sepBy1 p sep@ parses /one/ or more occurrences of @p@, separated
    -- by @sep@. Returns a list of values returned by @p@.

    sepBy1 :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]
    sepBy1 p sep        = do{ x <- p
                            ; xs <- many (sep >> p)
                            ; return (x:xs)
                            }

    -- | @sepEndBy1 p sep@ parses /one/ or more occurrences of @p@,
    -- separated and optionally ended by @sep@. Returns a list of values
    -- returned by @p@.

    sepEndBy1 :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]
    sepEndBy1 p sep     = do{ x <- p
                            ; do{ sep
                                ; xs <- sepEndBy p sep
                                ; return (x:xs)
                                }
                              <|> return [x]
                            }

    -- | @sepEndBy p sep@ parses /zero/ or more occurrences of @p@,
    -- separated and optionally ended by @sep@, ie. haskell style
    -- statements. Returns a list of values returned by @p@.
    --
    -- >  haskellStatements  = haskellStatement `sepEndBy` semi

    sepEndBy :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]
    sepEndBy p sep      = sepEndBy1 p sep <|> return []

    -- | @endBy1 p sep@ parses /one/ or more occurrences of @p@, seperated
    -- and ended by @sep@. Returns a list of values returned by @p@.

    endBy1 :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]
    endBy1 p sep        = many1 (do{ x <- p; sep; return x })

    -- | @endBy p sep@ parses /zero/ or more occurrences of @p@, seperated
    -- and ended by @sep@. Returns a list of values returned by @p@.
    --
    -- >   cStatements  = cStatement `endBy` semi

    endBy :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]
    endBy p sep         = many (do{ x <- p; sep; return x })

    -- | @count n p@ parses @n@ occurrences of @p@. If @n@ is smaller or
    -- equal to zero, the parser equals to @return []@. Returns a list of
    -- @n@ values returned by @p@.

    count :: (Stream s m t) => Int -> ParsecT s u m a -> ParsecT s u m [a]
    count n p           | n <= 0    = return []
                        | otherwise = sequence (replicate n p)

    -- | @chainr p op x@ parser /zero/ or more occurrences of @p@,
    -- separated by @op@ Returns a value obtained by a /right/ associative
    -- application of all functions returned by @op@ to the values returned
    -- by @p@. If there are no occurrences of @p@, the value @x@ is
    -- returned.

    chainr :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m (a -> a -> a) -> a -> ParsecT s u m a
    chainr p op x       = chainr1 p op <|> return x

    -- | @chainl p op x@ parser /zero/ or more occurrences of @p@,
    -- separated by @op@. Returns a value obtained by a /left/ associative
    -- application of all functions returned by @op@ to the values returned
    -- by @p@. If there are zero occurrences of @p@, the value @x@ is
    -- returned.

    chainl :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m (a -> a -> a) -> a -> ParsecT s u m a
    chainl p op x       = chainl1 p op <|> return x

    -- | @chainl1 p op x@ parser /one/ or more occurrences of @p@,
    -- separated by @op@ Returns a value obtained by a /left/ associative
    -- application of all functions returned by @op@ to the values returned
    -- by @p@. . This parser can for example be used to eliminate left
    -- recursion which typically occurs in expression grammars.
    --
    -- >  expr    = term   `chainl1` addop
    -- >  term    = factor `chainl1` mulop
    -- >  factor  = parens expr <|> integer
    -- >
    -- >  mulop   =   do{ symbol "*"; return (*)   }
    -- >          <|> do{ symbol "/"; return (div) }
    -- >
    -- >  addop   =   do{ symbol "+"; return (+) }
    -- >          <|> do{ symbol "-"; return (-) }

    chainl1 :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m (a -> a -> a) -> ParsecT s u m a
    chainl1 p op        = do{ x <- p; rest x }
                        where
                          rest x    = do{ f <- op
                                        ; y <- p
                                        ; rest (f x y)
                                        }
                                    <|> return x

    -- | @chainr1 p op x@ parser /one/ or more occurrences of |p|,
    -- separated by @op@ Returns a value obtained by a /right/ associative
    -- application of all functions returned by @op@ to the values returned
    -- by @p@.

    chainr1 :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m (a -> a -> a) -> ParsecT s u m a
    chainr1 p op        = scan
                        where
                          scan      = do{ x <- p; rest x }

                          rest x    = do{ f <- op
                                        ; y <- scan
                                        ; return (f x y)
                                        }
                                    <|> return x

    -----------------------------------------------------------
    -- Tricky combinators
    -----------------------------------------------------------
    -- | The parser @anyToken@ accepts any kind of token. It is for example
    -- used to implement 'eof'. Returns the accepted token.

    anyToken :: (Stream s m t, Show t) => ParsecT s u m t
    anyToken            = tokenPrim show (\pos _tok _toks -> pos) Just

    -- | This parser only succeeds at the end of the input. This is not a
    -- primitive parser but it is defined using 'notFollowedBy'.
    --
    -- >  eof  = notFollowedBy anyToken <?> "end of input"

    eof :: (Stream s m t, Show t) => ParsecT s u m ()
    eof                 = notFollowedBy anyToken <?> "end of input"

    -- | @notFollowedBy p@ only succeeds when parser @p@ fails. This parser
    -- does not consume any input. This parser can be used to implement the
    -- \'longest match\' rule. For example, when recognizing keywords (for
    -- example @let@), we want to make sure that a keyword is not followed
    -- by a legal identifier character, in which case the keyword is
    -- actually an identifier (for example @lets@). We can program this
    -- behaviour as follows:
    --
    -- >  keywordLet  = try (do{ string "let"
    -- >                       ; notFollowedBy alphaNum
    -- >                       })

    notFollowedBy :: (Stream s m t, Show a) => ParsecT s u m a -> ParsecT s u m ()
    notFollowedBy p     = try (do{ c <- try p; unexpected (show c) }
                               <|> return ()
                              )

    -- | @manyTill p end@ applies parser @p@ /zero/ or more times until
    -- parser @end@ succeeds. Returns the list of values returned by @p@.
    -- This parser can be used to scan comments:
    --
    -- >  simpleComment   = do{ string "<!--"
    -- >                      ; manyTill anyChar (try (string "-->"))
    -- >                      }
    --
    --    Note the overlapping parsers @anyChar@ and @string \"-->\"@, and
    --    therefore the use of the 'try' combinator.

    manyTill :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m end -> ParsecT s u m [a]
    manyTill p end      = scan
                        where
                          scan  = do{ end; return [] }
                                <|>
                                  do{ x <- p; xs <- scan; return (x:xs) }