Text.Parsec.Combinator

Commonly used generic combinators.

See also the parser-combinators package for additional (and generalised) combinators.

Documentation

choice :: Stream s m t => [ParsecT s u m a] -> ParsecT s u m a

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.

count :: Stream s m t => Int -> ParsecT s u m a -> ParsecT s u m [a]

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.

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 parses open, followed by p and close. Returns the value returned by p.

 braces  = between (symbol "{") (symbol "}")

option :: Stream s m t => a -> ParsecT s u m a -> ParsecT s u m a

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)
                         })

optionMaybe :: Stream s m t => ParsecT s u m a -> ParsecT s u m (Maybe a)

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.

optional :: Stream s m t => ParsecT s u m a -> ParsecT s u m ()

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.

skipMany1 :: Stream s m t => ParsecT s u m a -> ParsecT s u m ()

skipMany1 p applies the parser p one or more times, skipping its result.

many1 :: Stream s m t => ParsecT s u m a -> ParsecT s u m [a]

many1 p applies the parser p one or more times. Returns a list of the returned values of p.

 word  = many1 letter

sepBy :: Stream s m t => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]

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 ",")

sepBy1 :: Stream s m t => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]

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

endBy :: Stream s m t => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]

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

endBy1 :: Stream s m t => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]

endBy1 p sep parses one or more occurrences of p, separated and ended by sep. Returns a list of values returned by p.

sepEndBy :: Stream s m t => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]

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

sepEndBy1 :: Stream s m t => ParsecT s u m a -> ParsecT s u m sep -> ParsecT s u m [a]

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

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 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.

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 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   =   do{ symbol "*"; return (*)   }
         <|> do{ symbol "/"; return (div) }

 addop   =   do{ symbol "+"; return (+) }
         <|> do{ symbol "-"; return (-) }

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 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.

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 x 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.

eof :: (Stream s m t, Show t) => ParsecT s u m ()

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"

notFollowedBy :: (Stream s m t, Show a) => ParsecT s u m a -> ParsecT s u m ()

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
                      })

NOTE: Currently, notFollowedBy exhibits surprising behaviour when applied to a parser p that doesn't consume any input; specifically

• notFollowedBy . notFollowedBy is not equivalent to lookAhead, and
• notFollowedBy eof never fails.

See haskell/parsec#8 for more details.

manyTill :: Stream s m t => ParsecT s u m a -> ParsecT s u m end -> ParsecT s u m [a]

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.

lookAhead :: Stream s m t => ParsecT s u m a -> ParsecT s u m a

lookAhead p parses p without consuming any input.

If p fails and consumes some input, so does lookAhead. Combine with try if this is undesirable.

anyToken :: (Stream s m t, Show t) => ParsecT s u m t

The parser anyToken accepts any kind of token. It is for example used to implement eof. Returns the accepted token.

Debugging

As a more comprehensive alternative for debugging Parsec parsers, there's also the parsec-free package.

parserTrace :: (Show t, Stream s m t) => String -> ParsecT s u m ()

parserTrace label is an impure function, implemented with Debug.Trace that prints to the console the remaining parser state at the time it is invoked. It is intended to be used for debugging parsers by inspecting their intermediate states.

*> parseTest (oneOf "aeiou"  >> parserTrace "label") "atest"
label: "test"
...

Since: 3.1.12.0

parserTraced :: (Stream s m t, Show t) => String -> ParsecT s u m b -> ParsecT s u m b

parserTraced label p is an impure function, implemented with Debug.Trace that prints to the console the remaining parser state at the time it is invoked. It then continues to apply parser p, and if p fails will indicate that the label has been backtracked. It is intended to be used for debugging parsers by inspecting their intermediate states.

*>  parseTest (oneOf "aeiou"  >> parserTraced "label" (oneOf "nope")) "atest"
label: "test"
label backtracked
parse error at (line 1, column 2):
...

Since: 3.1.12.0