# Masala Parser: Javascript Parser Combinators

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Masala Parser is inspired by the paper titled:
[Direct Style Monadic Parser Combinators For The Real World](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/parsec-paper-letter.pdf).

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**
* It can extract data from a big text and **replace complex regexp**
* It works in any **browser**
* There is a good **typescript** type declaration
* It can validate complete structure with **variations**
* It's a great starting point for parser education. It's **way simpler than Lex & Yacc**.
* It's designed to be written in other languages (Python, Java, Rust) with the same interface

Masala Parser keywords are **simplicity**, **variations** and **maintainability**. You won't
need theoretical bases on languages for extraction or validation use cases.

Masala Parser has relatively good performances, however, Javascript is obviously not the fastest machine.

# Usage

With Node Js or modern build        
        
        npm install -S @masala/parser

Or in the browser 

* [download Release](https://github.com/d-plaindoux/masala-parser/releases)
* `<script src="masala-parser.min.js"/>`

Check the [Change Log](./changelog.md) if you can from a previous version.

# Reference

You will find an [Masala Parser online reference](http://www.robusta.io/masala-parser/ts/modules/_masala_parser_d_.html), generated from typescript interface.

# Quick Examples

## Hello World

```js
const helloParser = C.string('hello');
const white = C.char(' ');
const worldParser = C.string('world');
const combinator = helloParser.then(white.rep()).then(worldParser);
``` 

## Floor notation

```js
// 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.number())      // we have ['|', 4.6], we drop '|'
    .then(C.char('|').drop())   // we have [4.6, '|'], we keep [4.6]
    .single() // we had [4.6], now just 4.6
    .map(x =>Math.floor(x));

// The parser parses 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.

The parser could fetch every name, ie two consecutive words starting with uppercase. 
The parser will read through the document and aggregate a Response,
 which contains a value and the current offset in the text.

This value will evolve when the parser will meet new characters, 
but also with some function calls, such as the `map()` function.

![](./documentation/parsec-monoid.png)



## The Response

By definition, a Parser takes text as an input, and the Response is a structure that represents your problem. 
After parsing, there are two subtypes of `Response`:
 
* `Accept` when it found something.    
* `Reject` if it could not.


```js

    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());

```


## Building the Parser, and execution  

Like a language, the parser is built then executed. With Masala, we build using other parsers.

```js
const helloParser = C.string('hello');
const white = C.char(' ');
const worldParser = C.char('world');
const combinator = helloParser.then(white.rep()).then(worldParser);
``` 

There is a compiling time when you combine your parser, and an execution time when the parser 
runs its `parse(stream)` function. You will have the `Response` after parsing. 


So after building, the parser is executed against a stream of token. 
For simplicity, we will use a stream of characters, which is a text :)
 


## Hello Gandhi

The goal is to check that we have Hello 'someone', then to grab that name

```js
// Plain old javascript
const {Streams,  C}= require('@masala/parser');

var helloParser = C.string("Hello")
                    .then(C.char(' ').rep())
                    .then(C.letters()) // succession of A-Za-z letters
                    .last();    // keeping previous letters

var value = helloParser.val("Hello Gandhi");  // val(x) is a shortcut for parse(Stream.ofString(x)).value;

assertEquals('Gandhi', value);
```




# Parser Combinations

Let's use a real example. We combine many functions that return a new Parser. And each new Parser
is a combination of Parsers given by the standard bundles or previous functions.

```js
import  {Streams, N,C, F} from '@masala/parser';

const blanks = ()=>C.char(' ').optrep();

function operator(symbol) {
    return blanks().drop()
        .then(C.char(symbol))   // '+' or '*'
        .then(blanks().drop())
        .single();
}

function sum() {
    return N.integer()
        .then(operator('+').drop())
        .then(N.integer())  // then(x) creates a tuple - here, one value was dropped
        .map(tuple => tuple.at(0) + tuple.at(1)); 
        
}

function multiplication() {
    return N.integer()
        .then(operator('*').drop())
        .then(N.integer())
        .array() // we can have access to the value of the tuple
        .map( ([left,right])=> left * right); // more modern js 
}

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 a higher-order ingredient made from a good combination of spices.

![](./documentation/images/curry-paste.jpg)

## Precedence

Precedence is a technical term for priority. Using:

```js
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 ! Moreover `+2` and `-2` are acceptable scalars. 

## try(x).or(y)


`or()` will often be used with `try()`, that makes [backtracking](https://en.wikipedia.org/wiki/Backtracking) 
: it saves the current offset, then tries an option. And as soon that it's not satisfied, it goes back to the original 
offset and use the parser inside the `.or(P)` expression.`.

 Like Haskell's Parsec, Masala Parser can parse infinite look-ahead grammars but
 performs best on predictive (LL[1]) grammars.

Let see how 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. So now we must test '*2' ;
                                                   Is it (multiplication())? No ;
                                                   or(scalar()) ? neither




# Recursion

Masala-Parser (like Parsec) is a top-down parser and doesn't like [Left Recursion](https://cs.stackexchange.com/a/9971).

However, it is a resolved problem for this kind of parsers, with a lot of documentation. You can read more on [recursion
with Masala](./documentation/recursion.md), and checkout examples on our Github repository 
( [simple recursion](https://github.com/d-plaindoux/masala-parser/blob/master/integration-npm/examples/recursion/aaab-lazy-recursion.js), 
or [calculous expressions](https://github.com/d-plaindoux/masala-parser/blob/master/integration-npm/examples/operations/plus-minus.js) ).



# Simple documentation of Core bundles

## Core Parser Functions

Here is a link for [Core functions documentation](./documentation/parser-core-functions.md).

It will explain `then()`, `drop()`, `map()`, `rep()`, `opt()` and other core functions of the Parser
with code examples.

### 

## The Chars Bundle

Example: 

```js
C.char('-')
    .then(C.letters())
    .then(C.char('-'))
// accepts  '-hello-' ; value is ['-','hello','-']
// reject '-hel lo-' because space is not a letter    
```

[General use](./documentation/chars-bundle.md)

* `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](./documentation/chars-bundle.md)
* `lettersAs(symbol)`: accepts many letters and returns a string
* `emoji()`: accept any emoji sequence. [Opened Issue](https://github.com/d-plaindoux/masala-parser/issues/86).
* `notChar(x)`: accept if next input is not `x`
* `char(x)`: accept if next input is `x`
* `charIn('xyz')`: accept if next input is `x`, `y` or `z`
* `charNotIn('xyz')`: accept if next input is not `x`, `y` or `z`
* `subString(length)`: accept any next *length* characters and returns the equivalent string
* `string(word)`: accept if next input is the given `word`  
* `stringIn(words)`: accept if next input is the given `words` [More here](./documentation/chars-bundle.md)
* `notString(word)`: accept if next input is *not* the given `word`
* `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

Other example:

```js
C.string('Hello')
    .then(C.char(' '))
    .then(C.lowerCase().rep().join(''))

// accepts Hello johnny ; value is ['Hello', ' ', 'johnny']
// rejects Hello Johnny : J is not lowercase ; no value
```

## The Numbers Bundle


* `number()`: accept any float number, such as -2.3E+24, and returns a float    
* `digit()`: accept any single digit, and returns a **number**
* `digits()`: accept many digits, and returns a **number**. Warning: it does not accept **+-** signs symbols.
* `integer()`: accept any positive or negative integer




## 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 to `otherParser` 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 **E**nd **O**f **S**tream
* `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
* `F.dropTo(string|stopParser)`: Will traverse the document **including** the *stop parser*
    

Others:

* `F.lazy(parser, ?params)`: 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(value)`: forces a returned value
* `F.error()`: returns an error. Parser will never be accepted
* `F.satisfy(predicate)`: check if condition is satisfied
* `F.startsWith(value)`: create a no-op parser with initial value 





## License

Copyright (C)2016-2020 Didier Plaindoux & Nicolas Zozol

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.