| Portability | unknown |
|---|---|
| Stability | experimental |
| Maintainer | felipe.lessa@gmail.com |
Data.Attoparsec.Text
Contents
Description
Simple, efficient combinator parsing for Text strings,
loosely based on the Parsec library.
- data Parser a
- data Result 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-textcan 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-textis gained via high-performance parsers such astakeWhileandstring. If you use complicated combinators that return lists of characters, there is less performance difference between the two libraries. - Unlike Parsec 3,
attoparsec-textdoes not support being used as a monad transformer. -
attoparsec-textis specialised to deal only with strictTextinput. 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 theLazymodule, which feeds lazy chunks to a regular parser. - Parsec parsers can produce more helpful error messages than
attoparsec-textparsers. This is a matter of focus:attoparsec-textavoids 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 Result r
The result of a parse.
Constructors
| Fail Text [String] String | The parse failed. The |
| 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 |
| Done Text r | The parse succeeded. The |
Typeclass instances
The Parser type is an instance of the following classes:
-
Monad, wherefailthrows an exception (i.e. fails) with an error message. -
FunctorandApplicative, which follow the usual definitions. -
MonadPlus, wheremzerofails (with no error message) andmplusexecutes the right-hand parser if the left-hand one fails. -
Alternative, which followsMonadPlus.
The Result type is an instance of Functor, where fmap
transforms the value in a Done result.
Running parsers
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.
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
|
| -> 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
eitherResult :: Result r -> Either String r
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 characters
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.
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 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.
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
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
Consume all remaining input and return it as a single string.
Text parsing
Match either a single newline character '\n', or a carriage
return followed by a newline character "\r\n".
Numeric parsers
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.
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.
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.