Data.Attoparsec

| attoparsec-0.8.1.0: Fast combinator parsing for bytestrings | Contents | Index
Data.Attoparsec |
Portability | unknown
Stability | experimental
Maintainer | bos@serpentine.com
Contents
Differences from Parsec Performance considerations Parser types Typeclass instances Running parsers Result conversion Combinators Parsing individual bytes Byte classes Efficient string handling State observation and manipulation functions
Description
Simple, efficient combinator parsing for `ByteString` strings, loosely based on the Parsec library.
Synopsis
data Parser a
data Result r
= Fail !ByteString [String] String
| Partial (ByteString -> Result r)
| Done !ByteString r
parse :: Parser a -> ByteString -> Result a
feed :: Result r -> ByteString -> Result r
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
(<?>) :: Parser a -> String -> Parser a
try :: Parser a -> Parser a
module Data.Attoparsec.Combinator
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 ()
inClass :: String -> Word8 -> Bool
notInClass :: String -> Word8 -> Bool
string :: ByteString -> Parser ByteString
skipWhile :: (Word8 -> Bool) -> Parser ()
take :: Int -> Parser ByteString
takeWhile :: (Word8 -> Bool) -> Parser ByteString
takeWhile1 :: (Word8 -> Bool) -> Parser ByteString
takeTill :: (Word8 -> Bool) -> Parser ByteString
endOfInput :: Parser ()
ensure :: Int -> Parser ()
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 really isn't much performance difference the two libraries. Unlike Parsec 3, Attoparsec does not support being used as a monad transformer. This is mostly a matter of the implementor not having needed that functionality. Attoparsec is specialised to deal only with strict `ByteString` input. Efficiency concernts 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. 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.
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
data Parser a
The `Parser` type is a monad.
Instances
Monad Parser |
Functor Parser |
MonadPlus Parser |
Applicative Parser |
Alternative Parser |
data Result r
The result of a parse.
Constructors
Fail !ByteString [String] String | The parse failed. The `ByteString` 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 (ByteString -> Result 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 !ByteString r | The parse succeeded. The `ByteString` is the input that had not yet been consumed (if any) when the parse succeeded.
Instances
Functor Result |
Show r => Show (Result r) |
Typeclass instances
The `Parser` type is an instance of the following classes: `Monad`, where `fail` throws an exception (i.e. fails) with an error message. `Functor` and Applicative, which follow the usual definitions. MonadPlus, where mzero fails (with no error message) and mplus executes the right-hand parser if the left-hand one fails. Alternative, which follows MonadPlus. The `Result` type is an instance of `Functor`, where `fmap` transforms the value in a `Done` result.
Running parsers
parse :: Parser a -> ByteString -> Result a
Run a parser and return its result.
feed :: Result r -> ByteString -> Result r
If a parser has returned a `Partial` result, supply it with more input.
parseWith
:: 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
(<?>)
:: Parser a |
-> String | the name to use if parsing fails
-> Parser a |
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 useful in cases where a parser might consume some input before failing, i.e. the parser needs arbitrary lookahead. The downside to using this combinator is that it can retain input for longer than is desirable.
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.
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`.
    digit = satisfy 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 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.
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.
State observation and manipulation functions
endOfInput :: Parser ()
Match only if all input has been consumed.
ensure :: Int -> Parser ()
Succeed only if at least `n` bytes of input are available.
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