Data.Attoparsec.Text

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

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 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 Text input. Efficiency concerns rule out both lists and lazy text. The usual use for lazy text 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 text, see the 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 Text 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 another string will resume parsing at the point where it was suspended. 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 Text.

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.

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 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 = c == '\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

type Parser = Parser Text

type Result = IResult Text

data IResult t r

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

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

Constructors

Fail t [String] String

The parse failed. The t 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 (t -> IResult t 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 t r

The parse succeeded. The t parameter is the input that had not yet been consumed (if any) when the parse succeeded.

Instances

Functor (IResult t)

(Show t, Show r) => Show (IResult t r)

(NFData t, NFData r) => NFData (IResult t r)

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

Compare two IResult values for equality.

If both IResults 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 -> Text -> Result a

Run a parser.

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

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

parseOnly :: Parser a -> Text -> Either String a

Run a parser that cannot be resupplied via a Partial result.

parseWith

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.

parseTest :: Show a => Parser a -> Text -> 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

module Data.Attoparsec.Combinator

Parsing individual characters

char :: Char -> Parser Char

Match a specific character.

anyChar :: Parser Char

Match any character.

notChar :: Char -> Parser Char

Match any character except the given one.

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

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

digit = satisfy isDigit
    where isDigit c = c >= '0' && c <= '9'

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

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 ()

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

skipDigit = skip isDigit
    where isDigit c = c >= '0' && c <= '9'

Lookahead

peekChar :: Parser (Maybe Char)

Match any character, 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.

peekChar' :: Parser Char

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

Special character parsers

digit :: Parser Char

Parse a single digit, as recognised by isDigit.

letter :: Parser Char

Parse a letter, as recognised by isAlpha.

space :: Parser Char

Parse a space character, as recognised by isSpace.

Character classes

inClass :: String -> Char -> Bool

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 -> Bool

Match any character not in a set.

Efficient string handling

string :: Text -> Parser Text

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 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.

stringCI :: Text -> Parser Text

Deprecated: this is very inefficient, use asciiCI instead

Satisfy a literal string, ignoring case.

Note: this function is currently quite inefficient. Unicode case folding can change the length of a string ("ß" becomes ss), which makes a simple, efficient implementation tricky. We have (for now) chosen simplicity over efficiency.

asciiCI :: Text -> Parser Text

Satisfy a literal string, ignoring case for characters in the ASCII range.

skipSpace :: Parser ()

Skip over white space.

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

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

scan :: s -> (s -> Char -> Maybe s) -> Parser Text

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 character 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 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.

take :: Int -> Parser Text

Consume exactly n characters of input.

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

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 Text

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 Text

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.

String combinators

The .*> and <*. combinators are intended for use with the OverloadedStrings language extension. They simplify the common task of matching a statically known string, then immediately parsing something else.

An example makes this easier to understand:

{-# LANGUAGE OverloadedStrings #-}

shoeSize = "Shoe size: " .*> decimal

If we were to try to use *> above instead, the type checker would not be able to tell which IsString instance to use for the text in quotes. We would have to be explicit, using either a type signature or the string parser.

(.*>) :: Text -> Parser a -> Parser a

Type-specialized version of *> for Text.

(<*.) :: Parser a -> Text -> Parser a

Type-specialized version of <* for Text.

Consume all remaining input

takeText :: Parser Text

Consume all remaining input and return it as a single string.

takeLazyText :: Parser Text

Consume all remaining input and return it as a single string.

Text parsing

endOfLine :: Parser ()

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

isEndOfLine :: Char -> Bool

A predicate that matches either a carriage return '\r' or newline '\n' character.

isHorizontalSpace :: Char -> Bool

A predicate that matches either a space ' ' or horizontal tab '\t' character.

Numeric parsers

decimal :: Integral a => Parser a

Parse and decode an unsigned decimal number.

hexadecimal :: (Integral a, Bits a) => Parser a

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 a

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

double :: Parser Double

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.

This function does not accept string representations of "NaN" or "Infinity".

data Number

A numeric type that can represent integers accurately, and floating point numbers to the precision of a Double.

Constructors

I !Integer

D !Double

Instances

Eq Number

Fractional Number

Data Number

Num Number

Ord Number

Real Number

RealFrac Number

Show Number

Typeable Number

NFData Number

number :: Parser Number

Parse a number, attempting to preserve both speed and precision.

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

Note: This function is almost ten times faster than rational. On integral inputs, it gives perfectly accurate answers, and on floating point inputs, it is slightly less accurate than rational.

This function does not accept string representations of "NaN" or "Infinity".

rational :: Fractional a => Parser a

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"

This function does not accept string representations of "NaN" or "Infinity".

scientific :: Parser Scientific

Parse a scientific number.

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