Data.Attoparsec.Text

attoparsec-text-0.8.5.1: Fast combinator parsing for texts

Portability | unknown
Stability | experimental
Maintainer | felipe.lessa@gmail.com

Data.Attoparsec.Text

Contents

- Differences from Parsec
- Incremental input
- Performance considerations
- Parser types
  - Typeclass instances
- Running parsers
  - Result conversion
- Combinators
- Parsing individual characters
  - Special character parsers
  - Character classes
- Efficient string handling
  - Consume all remaining input
- Text parsing
- Numeric parsers
- State observation and manipulation functions
- Applicative specializations

Description

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

Synopsis

- data Parser a
- data Result r
  - = Fail Text [String] String
  - | Partial (Text -> Result r)
  - | Done Text r
- parse :: Parser a -> Text -> Result a
- feed :: Result r -> Text -> Result r
- parseOnly :: Parser a -> Text -> Either String a
- parseWith :: Monad m => m Text -> Parser a -> Text -> m (Result a)
- parseTest :: Show a => Parser a -> Text -> 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
- char :: Char -> Parser Char
- anyChar :: Parser Char
- notChar :: Char -> Parser Char
- satisfy :: (Char -> Bool) -> Parser Char
- satisfyWith :: (Char -> a) -> (a -> Bool) -> Parser a
- skip :: (Char -> Bool) -> Parser ()
- digit :: Parser Char
- letter :: Parser Char
- space :: Parser Char
- inClass :: String -> Char -> Bool
- notInClass :: String -> Char -> Bool
- string :: Text -> Parser Text
- skipSpace :: Parser ()
- skipWhile :: (Char -> Bool) -> Parser ()
- take :: Int -> Parser Text
- scan :: s -> (s -> Char -> Maybe s) -> Parser Text
- takeWhile :: (Char -> Bool) -> Parser Text
- takeWhile1 :: (Char -> Bool) -> Parser Text
- takeTill :: (Char -> Bool) -> Parser Text
- takeText :: Parser Text
- takeLazyText :: Parser Text
- endOfLine :: Parser ()
- decimal :: Integral a => Parser a
- hexadecimal :: Integral a => Parser a
- signed :: Num a => Parser a -> Parser a
- double :: Parser Double
- rational :: RealFloat a => Parser a
- endOfInput :: Parser ()
- atEnd :: Parser Bool
- (<*.) :: Applicative f => f a -> f Text -> f a
- (.*>) :: Applicative f => f Text -> f a -> f a

# 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 is less performance difference between the two libraries.
- Unlike Parsec 3, `attoparsec-text` does not support being used as a monad transformer.
- `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. If you must use lazy texts, see the `Lazy` module, which feeds lazy chunks to a regular parser.
- 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.

# Incremental input

`attoparsec-text` 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 text 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

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 Parser a

The `Parser` type is a monad.

Instances

Monad Parser |
Functor Parser |
MonadPlus Parser |
Applicative Parser |
Alternative Parser |
IsString (Parser Text) |
Monoid (Parser a) |

data Result r

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

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 -> 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

(<?>)

Arguments

:: 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 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.

    import Data.Char (isDigit)
    digit = satisfy isDigit

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`.

    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 Char

Parse a single digit.

letter :: Parser Char

Parse a single letter.

space :: Parser Char

Parse a space character.

## 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 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 ()

Skip over white space.

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

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

take :: Int -> Parser Text

Consume exactly `n` characters of input.

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.

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.

## 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"`.

# Numeric parsers

decimal :: Integral a => Parser a

Parse and decode an unsigned decimal number.

hexadecimal :: Integral 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.

rational :: RealFloat 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"

# State observation and manipulation functions

endOfInput :: Parser ()

Match only if all input has been consumed.

atEnd :: Parser Bool

Return an indication of whether the end of input has been reached.

# 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 a

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

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

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