Masala Parser: Javascript Parser Combinators
Masala Parser is inspired by the paper titled: Direct Style Monadic Parser Combinators For The Real World.
Masala Parser is a Javascript implementation of the Haskell Parsec. It is plain Javascript that works in the browser, is tested with more than 450 unit tests, covering 100% of code lines.
Use cases
- It can create a full parser from scratch as an alternative for Lex & yacc
- It can extract data from a big text and replace complex regexp
- It can validate complete structure with variations
- It can parse and execute custom operations
Masala Parser keywords are variations and maintainability. You won't need theoretical bases on languages for extraction or validation use cases.
Usage
With Node Js or modern build
npm install -S @masala/parser
Or in the browser
- download Release
<script src="masala-parser.min.js"/>
Check the Change Log if you can from a previous version.
Quick Examples
Floor notation
// N: Number Bundle, C: Chars Bundle
const {Streams, N, C}= require('@masala/parser');
const stream = Stream.ofString('|4.6|');
const floorCombinator = C.char('|').drop()
.then(N.numberLiteral) // we have ['|',4.6], we keep 4.6
.then(C.char('|').drop()) // we have [4.6, '|'], we keep 4.6
.map(x =>Math.floor(x));
// Parsec needs a stream of characters
const parsing = floorCombinator.parse(stream);
assertEquals( 4, parsing.value, 'Floor parsing');
Explanations
According to Wikipedia "in functional programming, a parser combinator is a higher-order function that accepts several parsers as input and returns a new parser as its output."
The Parser
Let's say we have a document :
The James Bond series, by writer Ian Fleming, focuses on a fictional British Secret Service agent created in 1953, who featured him in twelve novels and two short-story collections. Since Fleming's death in 1964, eight other authors have written authorised Bond novels or novelizations: Kingsley Amis, Christopher Wood, John Gardner, Raymond Benson, Sebastian Faulks, Jeffery Deaver, William Boyd and Anthony Horowitz.
There are many way to analyze this document, for example finding names inside. But what is a name ? We can say that it is a combination of two following words starting with an uppercase. But what is a word ? What are following words ? What is a starting uppercase word ?
The goal of a parser is to find out. The goal of Parsec is to make this easy.
The Monoid structure
A monoid is an object with functions and one single encapsulated value. Have you heard of jQuery ? The $
object is a monoid, where
the value is the DOM selection.
The parser will read through the document and aggregate values. The single value of the monoid will be modified by the document stream,
but can also be modified by function calls, such as the map()
function. The value is the Response
of your Parser
.
A Http Promise is also a good example. It will give you later the value. Masala does the same: it will give you
the Response
after parsing.
Hello 'X'
The goal is check that we have Hello 'something', then to grab that something
// Plain old javascript
const {Streams, C}= require('@masala/parser');
var helloParser = C.string("Hello")
.then(C.char(' ').rep())
.then(C.char(`'`)).drop()
.then(C.letter.rep()) // keeping repeated ascii letters
.then(C.char(`'`).drop()); // keeping previous letters
var parsing = helloParser.parse(Streams.ofString("Hello 'World'"));
// C.letter.rep() will give an array of letters
assertArrayEquals(['W','o','r','l','d'], parsing.value.array(), "Hello World joined");
Improvement with Extractor Bundle
We have used a complex combinator that shows us how to parse character by character. But you can build or use higher level parsers to do the same job. Masala Parser offers an Extractor Bundle that could replace all of your regexp extractions.
import {Streams, X} from '@masala/parser'
const line = Streams.ofString("Hello 'World'");
// Adding a `'` as a word separator;
const x = new X({moreSeparators: `'`});
const helloParser = x.words(false) // false because we don't keep separators
.map(x.last); // We had "Hello" and "World"
const value = helloParser.parse(line).value;
test.equals(value, 'World');
Parser Combinations
Let's use a real example. We combine many functions that returns a new Parser. And each new Parser is a combination of Parsers given by the standard bundles or previous functions.
import {Streams, N,C, F} from '@masala/parser';
const blanks = ()=>C.char(' ').optrep();
function operator(symbol) {
return blanks().thenRight(C.char(symbol)).thenLeft(blanks());
}
function sum() {
return N.integer.thenLeft(operator('+')).then(N.integer)
.map(values => values[0] + values[1]);
}
function multiplication() {
return N.integer.thenLeft(operator('*')).then(N.integer)
.map(values => values[0] * values[1]);
}
function scalar() {
return N.integer;
}
function combinator() {
return F.try(sum())
.or(F.try(multiplication())) // or() will often work with try()
.or(scalar());
}
function parseOperation(line) {
return combinator().parse(Streams.ofString(line));
}
assertEquals(4, parseOperation('2 +2').value, 'sum: ');
assertEquals(6, parseOperation('2 * 3').value, 'multiplication: ');
assertEquals(8, parseOperation('8').value, 'scalar: ');
A curry paste is an higher order ingredient made from a good combination of spices.
Precedence
Precedence is a technical term for priority. Using:
function combinator() {
return F.try(sum())
.or(F.try(multiplication())) // or() will often work with try()
.or(scalar());
}
console.info('sum: ',parseOperation('2+2').value);
We will give priority to sum, then multiplication, then scalar. If we had put scalar()
first, we would have first
accepted 2
, then what could we do with +2
alone ? It's not a valid sum !
try(x).or(y)
or()
will often be used with try()
. Like Haskell's Parsec, Masala-Parser can parse infinite look-ahead grammars but
performs best on predictive (LL[1]) grammars.
With try()
, we can look a bit ahead of next characters, then go back:
F.try(sum()).or(F.try(multiplication())).or(scalar())
// try(sum()) parser in action
2 *2
..ok..ok ↑oups: go back and try multiplication. Should be OK.
Suppose we do not try()
but use or()
directly:
sum().or(multiplication()).or(scalar())
// testing sum()
2 *2
..ok..ok ↑oups: cursor is not going back. Having now to test '*2' ;
Is it (multiplication())? No ; or(scalar()) ? neither
try()
has some benefits, but costs more in memory and CPU, as you test things twice.
You should avoid long sequences of try()
if memory is constrained. If possible, you can use or()
without try()
when there is no starting ambiguity.
N.integer.or(C.letter())
doesn't require a try()
.
Simple documentation of Core bundles
Core Parser Functions
Here is a link for Core functions documentation.
It will explain then()
, drop()
, map()
, rep()
, opt()
and other core functions of the Parser
with code examples.
The Response
A Parser can parse. When it finished this work, it can return two subtypes of Response
:
-
Accept
when it found something. -
Reject
if it could not.
let response = C.char('a').rep().parse(Streams.ofString('aaaa'));
assertEquals(response.value.join(''), 'aaaa' );
assertEquals(response.offset, 4 );
assertTrue(response.isAccepted());
assertTrue(response.isConsumed());
// Partially accepted
response = C.char('a').rep().parse(Streams.ofString('aabb'));
assertEquals(response.value.join(''), 'aa' );
assertEquals(response.offset, 2 );
assertTrue(response.isAccepted());
assertFalse(response.isConsumed());
The Flow Bundle
The flow bundle will mix ingredients together.
For example if you have a Parser p
, F.not(p)
will accept anything
that does not satisfy p
All of these functions will return a brand new Parser that you can combine with others.
Most important:
-
F.try(parser).or(otherParser)
: Try a parser and come back tootherParser
if failed -
F.any
: Accept any character (and so moves the cursor) -
F.not(parser)
: Accept anything that is not a parser. Often used to accept until a given stop -
F.eos
: Accepted if the Parser has reached the End Of Stream -
F.moveUntil(string|stopParser)
: Alternative for regex. Will traverse the document until the stop parser- returns
undefined
if stop is not found - returns all characters if stop is found, and set the cursor at the spot of the stop
- returns
-
F.dropTo(string|stopParser)
: Will traverse the document including the stop parser
Others:
-
F.lazy
: Makes a lazy evaluation. May be used for Left recursion (difficult) -
F.parse(parserFunction)
: Create a new Parser from a function. Usually, you won't start here. -
F.subStream(length)
: accept any next characters -
F.returns
: forces a returned value -
F.error
: returns an error. Parser will never be accepted -
F.satisfy
: check if condition is satisfied -
F.startsWith(value)
: create a no-op parser with initial value
The Chars Bundle
-
letter
: accept a european letter (and moves the cursor) -
letters
: accepts many letters and returns a string -
letterAs(symbol)
: accepts a european(default), ascii, or utf8 Letter. More here -
lettersAs(symbol)
: accepts many letters and returns a string -
emoji
: accept any emoji sequence. Opened Issue. -
notChar(x)
: accept if next input is notx
-
char(x)
: accept if next input isx
-
charIn('xyz')
: accept if next input isx
,y
orz
-
charNotIn('xyz')
: accept if next input is notx
,y
orz
-
subString(length)
: accept any next length characters and returns the equivalent string -
string(word)
: accept if next input is the givenword
-
stringIn(words)
: accept if next input is the givenwords
More here -
notString(word)
: accept if next input is not the givenword
-
charLiteral
: single quoted char element in C/Java :'a'
is accepted -
stringLiteral
: double quoted string element in java/json:"hello world"
is accepted -
lowerCase
: accept any next lower case inputs -
upperCase
: accept any next uppercase inputs
The Numbers Bundle
-
numberLiteral
: accept any float number, such as -2.3E+24, and returns a float -
digit
: accept any single digit, and return a single char (or in fact string, it's just javascript) -
digits
: accept many digits, and return a string. Warning: it does not accept +- signs symbols. -
integer
: accept any positive or negative integer
The Standard bundles
Masala Parser offers a generic Token Bundle, a data Extractor, a Json parser, and an experimental and incomplete markdown parser.
The Token Bundle
-
email
: accept a very large number of emails -
date
: accept a very small number of dates (2017-03-27 or 27/03/2017) -
blank(nothing|string|parser)
: accept standard blanks (space, tab), or defined characters, or a combined Parser -
eol
: accept End Of Line\n
or\r\n
The Extractor Bundle
The Extractor will help you to find valuable data in complex text (emails sent by platforms, website crawling...)
Parser
is a class
, but you never use new Parser()
. Other bundles are simple JS objects. The X
extractor is a
class to make customization easy. So you can extend it to override methods, or use its constructor to change options.
X constructor
const x = new X(options)
with default options to:
{
spacesCharacters:' \n',
wordSeparators:C.charIn(' \n:-,;'),
letter : C.letter,
moreSeparators: null
}
-
spacesCharacters
: series of chars. Usex.spaces()
to accept given spaces -
wordSeparators
: Parser. Usex.words()
to select words separated withwordSeparators
-
letter
: Parser. OriginalC.letter
are occidental letters. See opened issue. -
moreSeparator
: series of chars. You don't have to redefinewordSeparators
when using{moreSeparator:'$£€'}
X functions
-
x.spaces()
: accept spaces defined inoptions.spacesCharacters
-
x.digits()
: accept many digits and returns a string -
x.word()
: accept a word that satisfies repetition ofoptions.letter
. Returns the word as a string -
x.words(keepSpaces=true)
: accept repetition of previous words. SetkeepSpaces=false
to removes spaces from result -
x.wordsIn(arrayOfStrings, keepSpaces = true)
: accept given words, separated by previously definedwordSeparators
-
x.first
,x.last
: mappers to pick first or last word- example:
x.words().map(x.first)
will pick the first word of the document
- example:
JSON Bundle and Markdown Bundle
The JSON bundle offers an easy to use JSON parser. Obviously you could use native JSON.parse()
function. So it's more
a source of examples to deal with array structure.
Warning: The Markdown bundle is under active development and will move a lot !
The Markdown parser will not compile Markdown in HTML, but it will gives you a Javascript object (aka JSON structure). The Markdown bundle offers a series of Markdown tokens to build your own meta-markdown parser.
Tokens are:
-
blank
: blanks in paragraphs, including single end of line -
eol
:\n
or\r\n
-
lineFeed
: At least two EOL -
fourSpacesBlock
: Four spaces or two tabs (will accept option for x spaces and/or y tabs) -
stop
: End of pure text -
pureText
: Pure text, which is inside italic or bold characters -
italic
: italic text between*pureText*
or_pureText_
-
bold
: bold text between**pureText**
-
code
: code text between`pureText`
(double backticks for escape not yet supported) -
text (pureTextParser)
: higher level of pureText, if you need to redefine what is pureText -
formattedSequence (pureText, stop)
: combination of pureText, italic, bold and code -
formattedParagraph
: formattedSequence separated by a lineFeed -
titleLine
:title\n===
ortitle\n---
variant of title -
titleSharp
:### title
variant of title -
title
: titleLine or titleSharp -
bulletLv1
: Level one bullet -
bulletLv2
: Level two bullet -
bullet
: Level one or two bullets -
codeLine
: Four spaces indented code block line
License
Copyright (C)2016-2017 D. Plaindoux.
This program is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with this program; see the file COPYING. If not, write to the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.