megaparsec/Text/Parsec/Combinator.hs

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-----------------------------------------------------------------------------
-- |
-- Module : Text.Parsec.Combinator
-- Copyright : (c) Daan Leijen 1999-2001, (c) Paolo Martini 2007
-- License : BSD-style (see the LICENSE file)
--
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-- Maintainer : derek.a.elkins@gmail.com
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-- 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.
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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)
-- > })
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option :: (Stream s m t) => a -> ParsecT s u m a -> ParsecT s u m a
option x p = p <|> return x
-- | @option p@ tries to apply parser @p@. If @p@ fails without
-- consuming input, it return 'Nothing', otherwise it returns
-- 'Just' the value returned by @p@.
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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@.
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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 "}")
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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.
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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 ()
-}
-- | @many p@ applies the parser @p@ /one/ or more times. Returns a
-- list of the returned values of @p@.
--
-- > word = many1 letter
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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]
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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]
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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]
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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]
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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]
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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]
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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@.
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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.
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chainr :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m (a -> a -> a) -> a -> ParsecT s u m a
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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
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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.
--
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-- > 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
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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
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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.
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anyToken :: (Stream s m t, Show t) => ParsecT s u m t
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anyToken = tokenPrim show (\pos _tok _toks -> pos) Just
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-- | 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"
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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
-- > })
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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) }
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<|> 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.
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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) }
-- | @lookAhead p@ parses @p@ without consuming any input.
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lookAhead :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m a
lookAhead p = do{ state <- getParserState
; x <- p
; setParserState state
; return x
}