attoparsec-0.12.1.2: Fast combinator parsing for bytestrings and text

Portability | unknown
Stability | experimental
Maintainer | bos@serpentine.com
Safe Haskell | None

Data.Attoparsec.ByteString

Contents

- Differences from Parsec
- Incremental input
- Performance considerations
- Parser types
- Running parsers
  - Result conversion
- Parsing individual bytes
  - Lookahead
  - Byte classes
- Efficient string handling
  - Consume all remaining input
- Combinators
- State observation and manipulation functions

Description

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

Synopsis

- type Parser = Parser ByteString
- type Result = IResult ByteString
- data IResult i r
  - = Fail i [String] String
  - | Partial (i -> IResult i r)
  - | Done i r
- compareResults :: (Eq i, Eq r) => IResult i r -> IResult i r -> Maybe Bool
- parse :: Parser a -> ByteString -> Result a
- feed :: Monoid i => IResult i r -> i -> IResult i r
- parseOnly :: Parser a -> ByteString -> Either String a
- parseWith :: Monad m => m ByteString -> Parser a -> ByteString -> m (Result a)
- parseTest :: Show a => Parser a -> ByteString -> IO ()
- maybeResult :: Result r -> Maybe r
- eitherResult :: Result r -> Either String r
- word8 :: Word8 -> Parser Word8
- anyWord8 :: Parser Word8
- notWord8 :: Word8 -> Parser Word8
- satisfy :: (Word8 -> Bool) -> Parser Word8
- satisfyWith :: (Word8 -> a) -> (a -> Bool) -> Parser a
- skip :: (Word8 -> Bool) -> Parser ()
- peekWord8 :: Parser (Maybe Word8)
- peekWord8' :: Parser Word8
- inClass :: String -> Word8 -> Bool
- notInClass :: String -> Word8 -> Bool
- string :: ByteString -> Parser ByteString
- skipWhile :: (Word8 -> Bool) -> Parser ()
- take :: Int -> Parser ByteString
- scan :: s -> (s -> Word8 -> Maybe s) -> Parser ByteString
- takeWhile :: (Word8 -> Bool) -> Parser ByteString
- takeWhile1 :: (Word8 -> Bool) -> Parser ByteString
- takeTill :: (Word8 -> Bool) -> Parser ByteString
- takeByteString :: Parser ByteString
- takeLazyByteString :: Parser ByteString
- try :: Parser i a -> Parser i a
- (<?>) :: Parser i a -> String -> Parser i a
- choice :: Alternative f => [f a] -> f a
- count :: Monad m => Int -> m a -> m [a]
- option :: Alternative f => a -> f a -> f a
- many' :: MonadPlus m => m a -> m [a]
- many1 :: Alternative f => f a -> f [a]
- many1' :: MonadPlus m => m a -> m [a]
- manyTill :: Alternative f => f a -> f b -> f [a]
- manyTill' :: MonadPlus m => m a -> m b -> m [a]
- sepBy :: Alternative f => f a -> f s -> f [a]
- sepBy' :: MonadPlus m => m a -> m s -> m [a]
- sepBy1 :: Alternative f => f a -> f s -> f [a]
- sepBy1' :: MonadPlus m => m a -> m s -> m [a]
- skipMany :: Alternative f => f a -> f ()
- skipMany1 :: Alternative f => f a -> f ()
- eitherP :: Alternative f => f a -> f b -> f (Either a b)
- match :: Parser a -> Parser (ByteString, a)
- endOfInput :: forall t. Chunk t => Parser t ()
- atEnd :: Chunk t => Parser t Bool

# 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 Data.Attoparsec.ByteString.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 a bytestring will resume parsing at the point where it was suspended, with the bytestring you supplied used as new input at the end of the existing input. 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.

Note: incremental input does not imply that attoparsec will release portions of its internal state for garbage collection as it proceeds. Its internal representation is equivalent to a single `ByteString`: if you feed incremental input to a parser, it will require memory proportional to the amount of input you supply. (This is necessary to support arbitrary backtracking.)

# Performance considerations

If you write an attoparsec-based parser carefully, it can be realistic to expect it to perform similarly to 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 i r

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

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

Constructors

Fail i [String] String | The parse failed. The `i` 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 (i -> IResult i r) | Supply this continuation with more input so that the parser can resume. To indicate that no more input is available, pass an empty string to the continuation. __Note__: if you get a `Partial` result, do not call its continuation more than once.
Done i r | The parse succeeded. The `i` parameter is the input that had not yet been consumed (if any) when the parse succeeded.

Instances

Functor (IResult i) |
(Show i, Show r) => Show (IResult i r) |
(NFData i, NFData r) => NFData (IResult i r) |

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

Compare two `IResult` values for equality.

If both `IResult`s 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 :: Monoid i => IResult i r -> i -> IResult i 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.

This function does not force a parser to consume all of its input. Instead, any residual input will be discarded. To force a parser to consume all of its input, use something like this:

    parseOnly (myParser <* endOfInput)

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.

# 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.

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

## Lookahead

peekWord8 :: Parser (Maybe Word8)

Match any byte, to perform lookahead. 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.

peekWord8' :: Parser Word8

Match any byte, to perform lookahead. Does not consume any input, but will fail if end of input has been reached.

## 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.

# Combinators

try :: Parser i a -> Parser i 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.

(<?>)

Arguments

:: Parser i a |
-> String | the name to use if parsing fails
-> Parser i a |

Name the parser, in case failure occurs.

choice :: Alternative f => [f a] -> f a

`choice ps` tries to apply the actions in the list `ps` in order, until one of them succeeds. Returns the value of the succeeding action.

count :: Monad m => Int -> m a -> m [a]

Apply the given action repeatedly, returning every result.

option :: Alternative f => a -> f a -> f a

`option x p` tries to apply action `p`. If `p` fails without consuming input, it returns the value `x`, otherwise the value returned by `p`.

     priority  = option 0 (digitToInt <$> digit)

many' :: MonadPlus m => m a -> m [a]

`many' p` applies the action `p` zero or more times. Returns a list of the returned values of `p`. The value returned by `p` is forced to WHNF.

      word  = many' letter

many1 :: Alternative f => f a -> f [a]

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

      word  = many1 letter

many1' :: MonadPlus m => m a -> m [a]

`many1' p` applies the action `p` one or more times. Returns a list of the returned values of `p`. The value returned by `p` is forced to WHNF.

      word  = many1' letter

manyTill :: Alternative f => f a -> f b -> f [a]

`manyTill p end` applies action `p` zero or more times until action `end` succeeds, and returns the list of values returned by `p`. This can be used to scan comments:

      simpleComment   = string "<!--" *> manyTill anyChar (string "-->")

(Note the overlapping parsers `anyChar` and `string "-->"`. While this will work, it is not very efficient, as it will cause a lot of backtracking.)

manyTill' :: MonadPlus m => m a -> m b -> m [a]

`manyTill' p end` applies action `p` zero or more times until action `end` succeeds, and returns the list of values returned by `p`. This can be used to scan comments:

      simpleComment   = string "<!--" *> manyTill' anyChar (string "-->")

(Note the overlapping parsers `anyChar` and `string "-->"`. While this will work, it is not very efficient, as it will cause a lot of backtracking.)

The value returned by `p` is forced to WHNF.

sepBy :: Alternative f => f a -> f s -> f [a]

`sepBy p sep` applies zero or more occurrences of `p`, separated by `sep`. Returns a list of the values returned by `p`.

     commaSep p  = p `sepBy` (symbol ",")

sepBy' :: MonadPlus m => m a -> m s -> m [a]

`sepBy' p sep` applies zero or more occurrences of `p`, separated by `sep`. Returns a list of the values returned by `p`. The value returned by `p` is forced to WHNF.

     commaSep p  = p `sepBy'` (symbol ",")

sepBy1 :: Alternative f => f a -> f s -> f [a]

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

     commaSep p  = p `sepBy1` (symbol ",")

sepBy1' :: MonadPlus m => m a -> m s -> m [a]

`sepBy1' p sep` applies one or more occurrences of `p`, separated by `sep`. Returns a list of the values returned by `p`. The value returned by `p` is forced to WHNF.

     commaSep p  = p `sepBy1'` (symbol ",")

skipMany :: Alternative f => f a -> f ()

Skip zero or more instances of an action.

skipMany1 :: Alternative f => f a -> f ()

Skip one or more instances of an action.

eitherP :: Alternative f => f a -> f b -> f (Either a b)

Combine two alternatives.

match :: Parser a -> Parser (ByteString, a)

Return both the result of a parse and the portion of the input that was consumed while it was being parsed.

# State observation and manipulation functions

endOfInput :: forall t. Chunk t => Parser t ()

Match only if all input has been consumed.

atEnd :: Chunk t => Parser t Bool

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