|
Data.Attoparsec | Portability | unknown | Stability | experimental | Maintainer | bos@serpentine.com |
|
|
|
|
|
Description |
Simple, efficient combinator parsing for ByteString strings,
loosely based on the Parsec library.
|
|
Synopsis |
|
|
|
|
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
|
|
|
The Parser type is a monad.
| Instances | |
|
|
|
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 | |
|
|
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
|
|
|
Run a parser and return its result.
|
|
|
If a parser has returned a Partial result, supply it with more
input.
|
|
|
:: Monad m | | => m ByteString | | -> Parser a | Initial 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.
|
|
|
|
Run a parser and print its result to standard output.
|
|
Result conversion
|
|
|
Convert a Result value to a Maybe value. A Partial result
is treated as failure.
|
|
|
Convert a Result value to an Either value. A Partial result
is treated as failure.
|
|
Combinators
|
|
|
|
|
|
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
|
|
|
Match a specific byte.
|
|
|
Match any byte.
|
|
|
Match any byte except the given one.
|
|
|
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
|
|
|
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.
|
|
|
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
|
|
|
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.
|
|
|
Match any byte not in a set.
|
|
Efficient string handling
|
|
|
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.
|
|
|
Skip past input for as long as the predicate returns True.
|
|
|
Consume exactly n bytes of input.
|
|
|
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.
|
|
|
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.
|
|
|
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
|
|
|
Match only if all input has been consumed.
|
|
|
Succeed only if at least n bytes of input are available.
|
|
Produced by Haddock version 2.6.0 |