replace-megaparsec: Stream editing with parsers

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Stream editing and find-and-replace with Megaparsec monadic parser combinators.


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Versions 1.0.0.0, 1.0.1.0, 1.1.0.0, 1.1.1.0, 1.1.2.0, 1.1.2.0, 1.1.3.0, 1.1.4.0, 1.1.5.0, 1.2.0.0, 1.2.1.0, 1.3.0.0, 1.3.1.0, 1.3.2.0, 1.4.0.0, 1.4.1.0, 1.4.2.0, 1.4.3.0, 1.4.4.0, 1.4.5.0, 1.5.0.0, 1.5.0.1
Change log CHANGELOG.md
Dependencies base (>=4.0 && <5.0), megaparsec [details]
License BSD-2-Clause
Author James Brock
Maintainer jamesbrock@gmail.com
Category Parsing
Home page https://github.com/jamesdbrock/replace-megaparsec
Bug tracker https://github.com/jamesdbrock/replace-megaparsec/issues
Source repo head: git clone https://github.com/jamesdbrock/replace-megaparsec.git
Uploaded by JamesBrock at 2019-09-10T12:51:31Z

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Readme for replace-megaparsec-1.1.2.0

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replace-megaparsec

Hackage Stackage Nightly Stackage LTS

replace-megaparsec is for finding text patterns, and also editing and replacing the found patterns. This activity is traditionally done with regular expressions, but replace-megaparsec uses megaparsec parsers instead for the pattern matching.

replace-megaparsec can be used in the same sort of “pattern capture” or “find all” situations in which one would use Python re.findall or Perl m//, or Unix grep.

replace-megaparsec can be used in the same sort of “stream editing” or “search-and-replace” situations in which one would use Python re.sub, or Perl s///, or Unix sed, or awk.

Why would we want to do pattern matching and substitution with parsers instead of regular expressions?

Examples

Try the examples in ghci by running cabal v2-repl in the replace-megaparsec/ root directory.

The examples depend on these imports.

import Replace.Megaparsec
import Text.Megaparsec
import Text.Megaparsec.Char
import Text.Megaparsec.Char.Lexer

Parsing with sepCap family of parser combinators

The following examples show how to match a pattern to a string of text and deconstruct the string of text by separating it into sections which match the pattern, and sections which don't match.

Pattern match, capture only the parsed result with sepCap

Separate the input string into sections which can be parsed as a hexadecimal number with a prefix "0x", and sections which can't.

let hexparser = chunk "0x" >> hexadecimal :: Parsec Void String Integer
parseTest (sepCap hexparser) "0xA 000 0xFFFF"
[Right 10,Left " 000 ",Right 65535]

Pattern match, capture only the matched text with findAll

Just get the strings sections which match the hexadecimal parser, throw away the parsed number.

let hexparser = chunk "0x" >> hexadecimal :: Parsec Void String Integer
parseTest (findAll hexparser) "0xA 000 0xFFFF"
[Right "0xA",Left " 000 ",Right "0xFFFF"]

Pattern match, capture the matched text and the parsed result with findAllCap

Capture the parsed hexadecimal number, as well as the string section which parses as a hexadecimal number.

let hexparser = chunk "0x" >> hexadecimal :: Parsec Void String Integer
parseTest (findAllCap hexparser) "0xA 000 0xFFFF"
[Right ("0xA",10),Left " 000 ",Right ("0xFFFF",65535)]

Pattern match, capture only the locations of the matched patterns

Find all of the sections of the stream which match a string of spaces. Print a list of the offsets of the beginning of every pattern match.

import Data.Either
let spaceoffset = getOffset <* space1 :: Parsec Void String Int
parseTest (return . rights =<< sepCap spaceoffset) " a  b  "
[0,2,5]

Pattern match balanced parentheses

Find the outer parentheses of all balanced nested parentheses. Here's an example of matching a pattern that can't be expressed by a regular expression. We can express the pattern with a recursive parser.

let parens :: Parsec Void String ()
    parens = do
        char '('
        manyTill
            (void (noneOf "()") <|> void parens)
            (char ')')
        return ()

parseTest (findAll parens) "(()) (()())"
[Right "(())",Left " ",Right "(()())"]

Edit text strings by running parsers with streamEdit

The following examples show how to search for a pattern in a string of text and then edit the string of text to substitute in some replacement text for the matched patterns.

Pattern match and replace with a constant

Replace all carriage-return-newline instances with newline.

streamEdit (chunk "\r\n") (const "\n") "1\r\n2\r\n"
"1\n2\n"

Pattern match and edit the matches

Replace alphabetic characters with the next character in the alphabet.

streamEdit (some letterChar) (fmap succ) "HAL 9000"
"IBM 9000"

Pattern match and maybe edit the matches, or maybe leave them alone

Find all of the string sections s which can be parsed as a hexadecimal number r, and if r≤16, then replace s with a decimal number. Uses the match combinator.

let hexparser = chunk "0x" >> hexadecimal :: Parsec Void String Integer
streamEdit (match hexparser) (\(s,r) -> if r <= 16 then show r else s) "0xA 000 0xFFFF"
"10 000 0xFFFF"

Pattern match and edit the matches with IO

import System.Environment
streamEditT (char '{' *> manyTill anySingle (char '}')) getEnv "- {HOME} -"
"- /home/jbrock -"

Context-sensitive pattern match and edit the matches

Capitalize the third letter in a string. The capthird parser searches for individual letters, and it needs to remember how many times it has run so that it can match successfully only on the third time that it finds a letter. To enable the parser to remember how many times it has run, we'll compose the parser with a State monad from the mtl package. (Run in ghci with cabal v2-repl -b mtl).

import qualified Control.Monad.State.Strict as MTL
import Control.Monad.State.Strict (get, put, evalState)
import Data.Char (toUpper)

let capthird :: ParsecT Void String (MTL.State Int) String
    capthird = do
        x <- letterChar
        i <- get
        put (i+1)
        if i==3 then return [x] else empty

flip evalState 1 $ streamEditT capthird (return . fmap toUpper) "a a a a a"
"a a A a a"

Alternatives

http://hackage.haskell.org/package/regex

http://hackage.haskell.org/package/pipes-parse

http://hackage.haskell.org/package/stringsearch

http://hackage.haskell.org/package/substring-parser

http://hackage.haskell.org/package/pcre-utils

http://hackage.haskell.org/package/template

http://hackage.haskell.org/package/regex-applicative

Hypothetically Asked Questions

  1. Is it fast?

    lol not really. sepCap is fundamentally about consuming the stream one token at a time while we try and fail to run a parser and then backtrack each time. That's a slow activity.

    Consider a 1 megabyte file that consists of "foo" every ten bytes:

           foo       foo       foo       foo       foo       foo ...
    

    We want to replace all the "foo" with "bar". We would expect sed to be about at the upper bound of speed for this task, so here are the perf results when we compare sed s/foo/bar/g to replace-megaparsec with some different stream types.

    Method perf task-clock
    sed 39 msec
    streamEdit String 793 msec
    streamEdit ByteString 513 msec
    streamEdit Text 428 msec
  2. Could we write this library for parsec?

    No, because the match combinator doesn't exist for parsec. (I can't find it anywhere. Can it be written?)

  3. Could we write this library for attoparsec?

    I think so, but I wouldn't expect much of a speed improvement, because again, sepCap is a fundamentally slow activity, and anyway megaparsec is as fast as attoparsec.