attoparsec-0.8.2.0: Fast combinator parsing for bytestringsSource codeContentsIndex
Data.Attoparsec
Portabilityunknown
Stabilityexperimental
Maintainerbos@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 Source
The Parser type is a monad.
show/hide Instances
data Result r Source
The result of a parse.
Constructors
Fail !ByteString [String] StringThe 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 rThe parse succeeded. The ByteString is the input that had not yet been consumed (if any) when the parse succeeded.
show/hide Instances
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 aSource
Run a parser and return its result.
feed :: Result r -> ByteString -> Result rSource
If a parser has returned a Partial result, supply it with more input.
parseWithSource
:: Monad m
=> m ByteString
-> Parser aInitial input for the parser.
-> ByteString
-> 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 ()Source
Run a parser and print its result to standard output.
Result conversion
maybeResult :: Result r -> Maybe rSource
Convert a Result value to a Maybe value. A Partial result is treated as failure.
eitherResult :: Result r -> Either String rSource
Convert a Result value to an Either value. A Partial result is treated as failure.
Combinators
(<?>)Source
::
=> Parser athe name to use if parsing fails
-> String
-> Parser a
try :: Parser a -> Parser aSource

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 Word8Source
Match a specific byte.
anyWord8 :: Parser Word8Source
Match any byte.
notWord8 :: Word8 -> Parser Word8Source
Match any byte except the given one.
satisfy :: (Word8 -> Bool) -> Parser Word8Source

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 aSource
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 ()Source

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 -> BoolSource

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 -> BoolSource
Match any byte not in a set.
Efficient string handling
string :: ByteString -> Parser ByteStringSource

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 ()Source
Skip past input for as long as the predicate returns True.
take :: Int -> Parser ByteStringSource
Consume exactly n bytes of input.
takeWhile :: (Word8 -> Bool) -> Parser ByteStringSource

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 ByteStringSource

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 ByteStringSource

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 ()Source
Match only if all input has been consumed.
ensure :: Int -> Parser ()Source
Succeed only if at least n bytes of input are available.
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