Data.Attoparsec.ByteString

Simple, efficient combinator parsing for ByteString strings, loosely based on the Parsec library.

Differences from Parsec

Compared to Parsec 3, Attoparsec makes several tradeoffs. It is not intended for, or ideal for, all possible uses.

• While Attoparsec can consume input incrementally, Parsec cannot. Incremental input is a huge deal for efficient and secure network and system programming, since it gives much more control to users of the library over matters such as resource usage and the I/O model to use.
• Much of the performance advantage of Attoparsec is gained via high-performance parsers such as takeWhile and string. If you use complicated combinators that return lists of bytes or characters, there is less performance difference between the two libraries.
• Unlike Parsec 3, Attoparsec does not support being used as a monad transformer.
• Attoparsec is specialised to deal only with strict ByteString input. Efficiency concerns rule out both lists and lazy bytestrings. The usual use for lazy bytestrings would be to allow consumption of very large input without a large footprint. For this need, Attoparsec's incremental input provides an excellent substitute, with much more control over when input takes place. If you must use lazy bytestrings, see the Lazy module, which feeds lazy chunks to a regular parser.
• Parsec parsers can produce more helpful error messages than Attoparsec parsers. This is a matter of focus: Attoparsec avoids the extra book-keeping in favour of higher performance.

Incremental input

Attoparsec supports incremental input, meaning that you can feed it a bytestring that represents only part of the expected total amount of data to parse. If your parser reaches the end of a fragment of input and could consume more input, it will suspend parsing and return a Partial continuation.

Supplying the Partial continuation with another bytestring will resume parsing at the point where it was suspended. You must be prepared for the result of the resumed parse to be another Partial continuation.

To indicate that you have no more input, supply the Partial continuation with an empty bytestring.

Remember that some parsing combinators will not return a result until they reach the end of input. They may thus cause Partial results to be returned.

If you do not need support for incremental input, consider using the parseOnly function to run your parser. It will never prompt for more input.

Performance considerations

If you write an Attoparsec-based parser carefully, it can be realistic to expect it to perform within a factor of 2 of a hand-rolled C parser (measuring megabytes parsed per second).

To actually achieve high performance, there are a few guidelines that it is useful to follow.

Use the ByteString-oriented parsers whenever possible, e.g. takeWhile1 instead of many1 anyWord8. There is about a factor of 100 difference in performance between the two kinds of parser.

For very simple byte-testing predicates, write them by hand instead of using inClass or notInClass. For instance, both of these predicates test for an end-of-line byte, but the first is much faster than the second:

endOfLine_fast w = w == 13 || w == 10
endOfLine_slow   = inClass "\r\n"

Make active use of benchmarking and profiling tools to measure, find the problems with, and improve the performance of your parser.

Parser types

type Parser = Parser ByteString

type Result = IResult ByteString

data IResult t r

The result of a parse. This is parameterised over the type t of string that was processed.

This type is an instance of Functor, where fmap transforms the value in a Done result.

Constructors

Fail t [String] String

The parse failed. The t parameter is the input that had not yet been consumed when the failure occurred. The [String] is a list of contexts in which the error occurred. The String is the message describing the error, if any.

Partial (t -> IResult t r)

Supply this continuation with more input so that the parser can resume. To indicate that no more input is available, use an empty string.

Done t r

The parse succeeded. The t parameter is the input that had not yet been consumed (if any) when the parse succeeded.

Instances

Functor (IResult t)

(Show t, Show r) => Show (IResult t r)

(NFData t, NFData r) => NFData (IResult t r)

compareResults :: (Eq t, Eq r) => IResult t r -> IResult t r -> Maybe Bool

Compare two IResult values for equality.

If both IResults are Partial, the result will be Nothing, as they are incomplete and hence their equality cannot be known. (This is why there is no Eq instance for IResult.)

Running parsers

parse :: Parser a -> ByteString -> Result a

Run a parser.

feed :: Result r -> ByteString -> Result r

If a parser has returned a Partial result, supply it with more input.

parseOnly :: Parser a -> ByteString -> Either String a

Run a parser that cannot be resupplied via a Partial result.

parseWith

Arguments

:: Monad m

=> m ByteString

An action that will be executed to provide the parser with more input, if necessary. The action must return an empty string when there is no more input available.

-> Parser a

-> ByteString

Initial input for the parser.

-> m (Result a)

Run a parser with an initial input string, and a monadic action that can supply more input if needed.

parseTest :: Show a => Parser a -> ByteString -> IO ()

Run a parser and print its result to standard output.

Result conversion

maybeResult :: Result r -> Maybe r

Convert a Result value to a Maybe value. A Partial result is treated as failure.

eitherResult :: Result r -> Either String r

Convert a Result value to an Either value. A Partial result is treated as failure.

Combinators

(<?>)

Arguments

:: Parser a

-> String

the name to use if parsing fails

-> Parser a

Name the parser, in case failure occurs.

try :: Parser a -> Parser a

Attempt a parse, and if it fails, rewind the input so that no input appears to have been consumed.

This combinator is provided for compatibility with Parsec. Attoparsec parsers always backtrack on failure.

module Data.Attoparsec.Combinator

Parsing individual bytes

word8 :: Word8 -> Parser Word8

Match a specific byte.

anyWord8 :: Parser Word8

Match any byte.

notWord8 :: Word8 -> Parser Word8

Match any byte except the given one.

peekWord8 :: Parser (Maybe Word8)

Match any byte. Returns Nothing if end of input has been reached. Does not consume any input.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

satisfy :: (Word8 -> Bool) -> Parser Word8

The parser satisfy p succeeds for any byte for which the predicate p returns True. Returns the byte that is actually parsed.

digit = satisfy isDigit
    where isDigit w = w >= 48 && w <= 57

satisfyWith :: (Word8 -> a) -> (a -> Bool) -> Parser a

The parser satisfyWith f p transforms a byte, and succeeds if the predicate p returns True on the transformed value. The parser returns the transformed byte that was parsed.

skip :: (Word8 -> Bool) -> Parser ()

The parser skip p succeeds for any byte for which the predicate p returns True.

skipDigit = skip isDigit
    where isDigit w = w >= 48 && w <= 57

Byte classes

inClass :: String -> Word8 -> Bool

Match any byte in a set.

vowel = inClass "aeiou"

Range notation is supported.

halfAlphabet = inClass "a-nA-N"

To add a literal '-' to a set, place it at the beginning or end of the string.

notInClass :: String -> Word8 -> Bool

Match any byte not in a set.

Efficient string handling

string :: ByteString -> Parser ByteString

string s parses a sequence of bytes that identically match s. Returns the parsed string (i.e. s). This parser consumes no input if it fails (even if a partial match).

Note: The behaviour of this parser is different to that of the similarly-named parser in Parsec, as this one is all-or-nothing. To illustrate the difference, the following parser will fail under Parsec given an input of "for":

string "foo" <|> string "for"

The reason for its failure is that the first branch is a partial match, and will consume the letters 'f' and 'o' before failing. In Attoparsec, the above parser will succeed on that input, because the failed first branch will consume nothing.

skipWhile :: (Word8 -> Bool) -> Parser ()

Skip past input for as long as the predicate returns True.

take :: Int -> Parser ByteString

Consume exactly n bytes of input.

scan :: s -> (s -> Word8 -> Maybe s) -> Parser ByteString

A stateful scanner. The predicate consumes and transforms a state argument, and each transformed state is passed to successive invocations of the predicate on each byte of the input until one returns Nothing or the input ends.

This parser does not fail. It will return an empty string if the predicate returns Nothing on the first byte of input.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

takeWhile :: (Word8 -> Bool) -> Parser ByteString

Consume input as long as the predicate returns True, and return the consumed input.

This parser does not fail. It will return an empty string if the predicate returns False on the first byte of input.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

takeWhile1 :: (Word8 -> Bool) -> Parser ByteString

Consume input as long as the predicate returns True, and return the consumed input.

This parser requires the predicate to succeed on at least one byte of input: it will fail if the predicate never returns True or if there is no input left.

takeTill :: (Word8 -> Bool) -> Parser ByteString

Consume input as long as the predicate returns False (i.e. until it returns True), and return the consumed input.

This parser does not fail. It will return an empty string if the predicate returns True on the first byte of input.

Note: Because this parser does not fail, do not use it with combinators such as many, because such parsers loop until a failure occurs. Careless use will thus result in an infinite loop.

Consume all remaining input

takeByteString :: Parser ByteString

Consume all remaining input and return it as a single string.

takeLazyByteString :: Parser ByteString

Consume all remaining input and return it as a single string.

State observation and manipulation functions

endOfInput :: Parser ()

Match only if all input has been consumed.

atEnd :: Parser Bool

Return an indication of whether the end of input has been reached.