attoparsec-text-0.8.2.0: Fast combinator parsing for texts

Portabilityunknown
Stabilityexperimental
Maintainerfelipe.lessa@gmail.com

Data.Attoparsec.Text

Contents

Description

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

Synopsis

Differences from Parsec

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

  • While attoparsec-text 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-text is gained via high-performance parsers such as takeWhile and string. If you use complicated combinators that return lists of characters, there really isn't much performance difference the two libraries.
  • Unlike Parsec 3, attoparsec-text does not support being used as a monad transformer. This is mostly a matter of the implementor not having needed that functionality.
  • attoparsec-text is specialised to deal only with strict Text input. Efficiency concernts rule out both lists and lazy texts. The usual use for lazy texts would be to allow consumption of very large input without a large footprint. For this need, attoparsec-text'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-text parsers. This is a matter of focus: attoparsec-text avoids the extra book-keeping in favour of higher performance.

Performance considerations

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

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

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

endOfLine_fast c = w == '\r' || c == '\n'
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 Result r Source

The result of a parse.

Constructors

Fail !Text [String] String

The parse failed. The Text 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 (Text -> 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 !Text r

The parse succeeded. The Text 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

parse :: Parser a -> Text -> Result aSource

Run a parser and return its result.

parseTest :: Show a => Parser a -> Text -> IO ()Source

Run a parser and print its result to standard output.

parseWithSource

Arguments

:: Monad m 
=> m Text

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 
-> Text

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.

feed :: Result r -> Text -> Result rSource

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

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

Arguments

:: Parser a 
-> String

the name to use if parsing fails

-> 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.

Parsing individual characters

char :: Char -> Parser CharSource

Match a specific character.

anyChar :: Parser CharSource

Match any character.

notChar :: Char -> Parser CharSource

Match any character except the given one.

satisfy :: (Char -> Bool) -> Parser CharSource

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

import Data.Char (isDigit)
digit = satisfy isDigit

satisfyWith :: (Char -> a) -> (a -> Bool) -> Parser aSource

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

skip :: (Char -> Bool) -> Parser ()Source

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

import Data.Char (isDigit)
digit = satisfy isDigit

Special character parsers

Special parser for characters. Unlike the original attoparsec package, these parsers do work correctly for all encodings. Internally Data.Char module is used.

digit :: Parser CharSource

Parse a single digit.

letter :: Parser CharSource

Parse a single letter.

space :: Parser CharSource

Parse a space character.

Character classes

inClass :: String -> Char -> BoolSource

Match any character 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 -> Char -> BoolSource

Match any character not in a set.

Efficient string handling

string :: Text -> Parser TextSource

string s parses a sequence of characters 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, both the original on bytestrings and this one on texts, the above parser will succeed on that input, because the failed first branch will consume nothing.

skipSpace :: Parser ()Source

Skip over white space.

skipWhile :: (Char -> Bool) -> Parser ()Source

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

take :: Int -> Parser TextSource

Consume exactly n characters of input.

takeWhile :: (Char -> Bool) -> Parser TextSource

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 character 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 :: (Char -> Bool) -> Parser TextSource

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 character of input: it will fail if the predicate never returns True or if there is no input left.

takeTill :: (Char -> Bool) -> Parser TextSource

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 character 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.

Text parsing

endOfLine :: Parser ()Source

Match either a single newline character '\n', or a carriage return followed by a newline character "\r\n".

Numeric parsers

decimal :: Integral a => Parser aSource

Parse and decode an unsigned decimal number.

hexadecimal :: Integral a => Parser aSource

Parse and decode an unsigned hexadecimal number. The hex digits 'a' through 'f' may be upper or lower case.

This parser does not accept a leading "0x" string.

signed :: Num a => Parser a -> Parser aSource

Parse a number with an optional leading '+' or '-' sign character.

double :: Parser DoubleSource

Parse a rational number.

The syntax accepted by this parser is the same as for rational.

Note: This function is almost ten times faster than rational, but is slightly less accurate.

The Double type supports about 16 decimal places of accuracy. For 94.2% of numbers, this function and rational give identical results, but for the remaining 5.8%, this function loses precision around the 15th decimal place. For 0.001% of numbers, this function will lose precision at the 13th or 14th decimal place.

rational :: RealFloat a => Parser aSource

Parse a rational number.

This parser accepts an optional leading sign character, followed by at least one decimal digit. The syntax similar to that accepted by the read function, with the exception that a trailing '.' or 'e' not followed by a number is not consumed.

Examples with behaviour identical to read, if you feed an empty continuation to the first result:

rational "3"     == Done 3.0 ""
rational "3.1"   == Done 3.1 ""
rational "3e4"   == Done 30000.0 ""
rational "3.1e4" == Done 31000.0 ""

Examples with behaviour identical to read:

rational ".3"    == Fail "input does not start with a digit"
rational "e3"    == Fail "input does not start with a digit"

Examples of differences from read:

rational "3.foo" == Done 3.0 ".foo"
rational "3e"    == Done 3.0 "e"

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 characters of input are available.

Applicative specializations

We provide specializations of <* and *> as <*. and .*>, respectively. Together with IsString instance of Parser, you may write parsers applicatively more easily. For example:

 paren p = "(" .*> p <*. ")"

instead of the more verbose

 paren p = string "(" *> p <* string ")"

(<*.) :: Applicative f => f a -> f Text -> f aSource

Same as Applicative's <* but specialized to Text on the second argument.

(.*>) :: Applicative f => f Text -> f a -> f aSource

Same as Applicative's *> but specialized to Text on the first argument.