megaparsec/Text/Megaparsec/Combinator.hs

-- |
-- Module      :  Text.Megaparsec.Combinator
-- Copyright   :  © 2015 Megaparsec contributors
--                © 2007 Paolo Martini
--                © 1999–2001 Daan Leijen
-- License     :  BSD3
--
-- Maintainer  :  Mark Karpov <markkarpov@opmbx.org>
-- Stability   :  experimental
-- Portability :  portable
--
-- Commonly used generic combinators.

module Text.Megaparsec.Combinator
    ( choice
    , count
    , between
    , option
    , optionMaybe
    , skipSome
    , sepBy
    , sepBy1
    , endBy
    , endBy1
    , sepEndBy
    , sepEndBy1
    , chainl
    , chainl1
    , chainr
    , chainr1
    , eof
    , notFollowedBy
    , manyTill
    , lookAhead
    , anyToken )
where

import Control.Applicative ((<|>), many, some)
import Control.Monad

import Text.Megaparsec.Prim
import Text.Megaparsec.ShowToken

-- | @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 = foldr (<|>) mzero

-- | @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 (digitToInt <$> digit)

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 (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 = (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 = open *> p <* close

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

skipSome :: Stream s m t => ParsecT s u m a -> ParsecT s u m ()
skipSome p = p *> skipMany p

-- | @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 = (:) <$> p <*> many (sep *> p)

-- | @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 = p >>= \x -> ((x:) <$> (sep *> sepEndBy p sep)) <|> return [x]

-- | @sepEndBy p sep@ parses /zero/ or more occurrences of @p@,
-- separated and optionally ended by @sep@, i.e. 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@, separated
-- 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 = some (p <* sep)

-- | @endBy p sep@ parses /zero/ or more occurrences of @p@, separated
-- 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 (p <* sep)

-- | @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 = replicateM n p

-- | @chainr p op x@ parses /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@ parses /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@ parses /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  = (symbol "*" >> return (*))
-- >      <|> (symbol "/" >> return (div))
-- >
-- > addop  = (symbol "+" >> return (+))
-- >      <|> (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 = p >>= rest
    where rest x = ((($ x) <$> op <*> p) >>= rest) <|> return x

-- | @chainr1 p op@ parses /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 = p >>= rest
    where rest x = (($ x) <$> op <*> chainr1 p op) <|> return x

-- | 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 => ParsecT s u m t
anyToken = tokenPrim (\pos _ _ -> 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 => 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 (string "let" >> notFollowedBy alphaNum)

notFollowedBy :: (Stream s m t, ShowToken a) =>
                 ParsecT s u m a -> ParsecT s u m ()
notFollowedBy p = try ((try p >>= (unexpected . showToken)) <|> 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 = string "<!--" >> manyTill anyChar (string "-->")
--
-- Note that although parsers @anyChar@ and @string \"-->\"@ overlap, the
-- combinator uses 'try' internally to parse @end@, so it's OK.

manyTill :: Stream s m t =>
            ParsecT s u m a -> ParsecT s u m end -> ParsecT s u m [a]
manyTill p end = (try end *> return []) <|> ((:) <$> p <*> manyTill p end)