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Programming language: Haskell
License: MIT License
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README

PariPari: Fast parser combinator library for Haskell

Hackage Build Status

PariPari is a parser combinator library for Haskell. PariPari can be used as a drop in replacement for the Parsec class of libraries. However be aware that the library is new and unstable.

PariPari offers two parsing strategies. There is a fast Acceptor and a slower Reporter. If the Acceptor fails, the Reporter returns a report about the parsing errors. This allows for fast parsing in the good case without compromising on the quality of the error messages.

Like Attoparsec, the parser combinators backtrack by default. To avoid exponential parsing time if errors are found and in order to avoid bad error messages, the <!> parser combinator is provided, which does not backtrack.

PariPari operates on strict ByteString and Text. As a consequence, PariPari is only a good fit for data which is available at once (no streaming). If characters are parsed using the char and satisfy combinators, bytestrings are interpreted as UTF-8 and decoded on the fly. The interface of PariPari matches mostly the one of Attoparsec/Megaparsec/etc.

Features

  • Fast-path parser without error reporting (Acceptor) and fallback to slower error reporting parser (Reporter) to optimize the common case
  • Backtracking by default for ease of use
  • <!> can be used to avoid backtracking
  • Support for strict UTF-8 ByteString and strict Text
  • Combinators for indentation-sensitive parsing
  • Error recovery support via recover
  • Provides flexible parsers for integers and fractional numbers of base 2 to 36 with support for separators between digits
  • Most Parsec/Megaparsec combinators provided, relying on the parser-combinators library

Example

In this example we use PariPari to parse JSON. The following is literate haskell.

Prologue

We specify a preprocessor, language pragmas and the library imports. Performance of PariPari depends crucially on the specialisation of Parser k a to Acceptor ByteString a and Reporter ByteString a. In larger parsers it seems that the GHC specialiser does not kick in. As a workaround we use paripari-specialise-all as a preprocessor, which generates SPECIALISE pragmas from our custom SPECIALISE_ALL pragma. Using the preprocessor is not necessary, however without it I observed 2x-4x slowdowns in parsing speed.

{-# OPTIONS_GHC -F -pgmF paripari-specialise-all #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE Rank2Types #-}

import Data.Foldable (for_)
import System.Environment (getArgs)
import Text.PariPari
import qualified Data.ByteString as B

Basic parser types

We parametrize the parser with the string type. Both ByteString and Text are supported. Note that even in the case of ByteStrings a Parser instance is provided, which interprets the bytes as UTF-8.

type StringType = B.ByteString
type PMonad p = Parser StringType p
type P a = (forall p. PMonad p => p a)

The P shortcut can be used for simple combinators. For functions returning parsers, the PMonad constraint must be used for specialization to work, e.g., char :: PMonad p => Char -> p Char.

Now we ensure that the GHC specialiser kicks in and specialises all parsers. These pragmas are processed by the preprocessor paripari-specialise-all.

{-# SPECIALISE_ALL PMonad p = p ~ Acceptor StringType #-}
{-# SPECIALISE_ALL PMonad p = p ~ Reporter StringType #-}
{-# SPECIALISE_ALL P = Acceptor StringType #-}
{-# SPECIALISE_ALL P = Reporter StringType #-}

JSON datatype

We define a datatype of JSON values.

data Value
  = Object ![(StringType, Value)]
  | Array  ![Value]
  | String !StringType
  | Number !Integer !Integer
  | Bool   !Bool
  | Null
  deriving (Eq, Show)

Parsers

A JSON toplevel value is either an object or an array.

json :: P Value
json = space *> (object <|> array) <?> "json"

Objects consist of pairs of a text string and a value.

object :: P Value
object = Object <$> (char '{' *> space *> sepBy pair (space *> char ',' *> space) <* space <* char '}') <?> "object"

pair :: P (StringType, Value)
pair = (,) <$> (text <* space) <*> (char ':' *> space *> value)

Arrays are a list of values.

array :: P Value
array = Array <$> (char '[' *> sepBy value (space *> char ',' *> space) <* space <* char ']') <?> "array"

Furthermore, JSON supports text strings, boolean values, null and floating point numbers.

value :: P Value
value =
  (String <$> text)
    <|> object
    <|> array
    <|> (Bool False <$ string "false")
    <|> (Bool True  <$ string "true")
    <|> (Null       <$ string "null")
    <|> number

text :: P StringType
text = char '"' *> takeCharsWhile (/= '"') <* char '"' <?> "text"

Floating point numbers are parsed by fractionDec which returns a coefficient, the base of the exponent and the exponent. The conversion to Double can be done for example by the scientific library.

number :: P Value
number = label "number" $ do
  neg <- sign
  frac <- fractionDec (pure ())
  pure $ case frac of
           Left n -> Number (neg n) 0
           Right (c, _, e) -> Number (neg c) e

For spaces we need another helper function.

space :: P ()
space = skipCharsWhile (\c -> c == ' ' || c == '\n' || c == '\t')

Main function

The main function of the example program reads a file, runs the parser and prints the value if the parsing succeeded. In the case of an error a report is printed.

main :: IO ()
main = do
  args <- getArgs
  case args of
    [file] -> do
      src <- B.readFile file
      let (result, reports) = runParser json file src
      for_ reports $ putStrLn . showReport
      print result
    _ -> error "Usage: paripari-example test.json"

Benchmark

See the repository parsers-bench.

parsers-bench-0.1.0: benchmarks
Running 1 benchmarks...
Benchmark bench-speed: RUNNING...
benchmarking CSV (PariPari)/csv-40.csv
time                 16.34 μs   (16.18 μs .. 16.53 μs)
                     0.998 R²   (0.996 R² .. 0.999 R²)
mean                 16.87 μs   (16.41 μs .. 18.64 μs)
std dev              2.703 μs   (569.0 ns .. 5.580 μs)
variance introduced by outliers: 94% (severely inflated)

benchmarking CSV (PariPari, Reporter)/csv-40.csv
time                 104.9 μs   (103.2 μs .. 106.5 μs)
                     0.985 R²   (0.972 R² .. 0.992 R²)
mean                 128.0 μs   (118.4 μs .. 140.0 μs)
std dev              40.73 μs   (30.44 μs .. 51.00 μs)
variance introduced by outliers: 98% (severely inflated)

benchmarking CSV (Attoparsec)/csv-40.csv
time                 57.69 μs   (56.52 μs .. 60.22 μs)
                     0.917 R²   (0.796 R² .. 0.988 R²)
mean                 68.62 μs   (62.95 μs .. 81.74 μs)
std dev              27.44 μs   (15.54 μs .. 49.65 μs)
variance introduced by outliers: 99% (severely inflated)

benchmarking CSV (Megaparsec)/csv-40.csv
time                 52.12 μs   (51.54 μs .. 52.98 μs)
                     0.996 R²   (0.993 R² .. 0.998 R²)
mean                 54.34 μs   (53.25 μs .. 56.03 μs)
std dev              4.705 μs   (3.416 μs .. 6.766 μs)
variance introduced by outliers: 79% (severely inflated)

benchmarking Log (PariPari)/log-40.log
time                 364.4 μs   (359.8 μs .. 369.6 μs)
                     0.998 R²   (0.997 R² .. 0.999 R²)
mean                 362.3 μs   (359.7 μs .. 365.9 μs)
std dev              10.50 μs   (7.760 μs .. 14.60 μs)
variance introduced by outliers: 22% (moderately inflated)

benchmarking Log (PariPari, Reporter)/log-40.log
time                 411.5 μs   (404.1 μs .. 421.8 μs)
                     0.984 R²   (0.964 R² .. 0.996 R²)
mean                 425.2 μs   (411.3 μs .. 453.2 μs)
std dev              60.17 μs   (37.11 μs .. 88.14 μs)
variance introduced by outliers: 87% (severely inflated)

benchmarking Log (Attoparsec)/log-40.log
time                 368.6 μs   (364.2 μs .. 372.5 μs)
                     0.998 R²   (0.997 R² .. 0.999 R²)
mean                 364.6 μs   (360.9 μs .. 371.3 μs)
std dev              16.87 μs   (10.96 μs .. 27.01 μs)
variance introduced by outliers: 42% (moderately inflated)

benchmarking Log (Megaparsec)/log-40.log
time                 412.4 μs   (405.9 μs .. 423.8 μs)
                     0.992 R²   (0.982 R² .. 0.998 R²)
mean                 419.7 μs   (410.9 μs .. 433.7 μs)
std dev              34.86 μs   (21.22 μs .. 51.70 μs)
variance introduced by outliers: 69% (severely inflated)

benchmarking JSON (PariPari)/json-40.json
time                 20.33 μs   (20.08 μs .. 20.68 μs)
                     0.998 R²   (0.997 R² .. 1.000 R²)
mean                 20.48 μs   (20.27 μs .. 21.02 μs)
std dev              1.179 μs   (578.2 ns .. 2.199 μs)
variance introduced by outliers: 65% (severely inflated)

benchmarking JSON (PariPari, Reporter)/json-40.json
time                 76.16 μs   (74.89 μs .. 78.25 μs)
                     0.994 R²   (0.987 R² .. 0.999 R²)
mean                 76.02 μs   (74.68 μs .. 78.57 μs)
std dev              5.598 μs   (3.438 μs .. 8.326 μs)
variance introduced by outliers: 72% (severely inflated)

benchmarking JSON (Attoparsec)/json-40.json
time                 21.67 μs   (21.43 μs .. 21.97 μs)
                     0.999 R²   (0.998 R² .. 0.999 R²)
mean                 21.69 μs   (21.48 μs .. 21.99 μs)
std dev              836.0 ns   (554.7 ns .. 1.371 μs)
variance introduced by outliers: 45% (moderately inflated)

benchmarking JSON (Megaparsec)/json-40.json
time                 31.50 μs   (30.84 μs .. 32.42 μs)
                     0.994 R²   (0.989 R² .. 0.998 R²)
mean                 31.45 μs   (30.88 μs .. 32.42 μs)
std dev              2.377 μs   (1.554 μs .. 3.610 μs)
variance introduced by outliers: 75% (severely inflated)

benchmarking JSON (PariPari, highlevel)/json-40.json
time                 30.63 μs   (30.08 μs .. 31.24 μs)
                     0.998 R²   (0.996 R² .. 0.999 R²)
mean                 30.69 μs   (30.35 μs .. 31.27 μs)
std dev              1.459 μs   (1.056 μs .. 2.053 μs)
variance introduced by outliers: 54% (severely inflated)

benchmarking JSON (PariPari, Reporter, highlevel)/json-40.json
time                 109.8 μs   (107.2 μs .. 113.2 μs)
                     0.995 R²   (0.992 R² .. 0.998 R²)
mean                 110.0 μs   (108.6 μs .. 112.2 μs)
std dev              6.079 μs   (4.499 μs .. 7.572 μs)
variance introduced by outliers: 57% (severely inflated)

benchmarking JSON (Attoparsec, highlevel)/json-40.json
time                 33.94 μs   (33.56 μs .. 34.39 μs)
                     0.998 R²   (0.997 R² .. 0.999 R²)
mean                 34.31 μs   (33.87 μs .. 35.74 μs)
std dev              2.511 μs   (896.2 ns .. 5.032 μs)
variance introduced by outliers: 74% (severely inflated)

benchmarking JSON (Megaparsec, highlevel)/json-40.json
time                 58.52 μs   (56.70 μs .. 60.51 μs)
                     0.993 R²   (0.990 R² .. 0.998 R²)
mean                 57.58 μs   (56.73 μs .. 58.92 μs)
std dev              3.511 μs   (2.575 μs .. 4.710 μs)
variance introduced by outliers: 64% (severely inflated)

Benchmark bench-speed: FINISH

Thanks

  • Mark Karpov @mrkkrp - For the parser-combinators library, which was extracted from Megaparsec and parsers-bench. The json example above is adapted from parsers-bench.

Related projects

License

Released under the MIT License.

Copyright (c) 2018 Daniel Mendler


*Note that all licence references and agreements mentioned in the paripari README section above are relevant to that project's source code only.