Ohm ·

Ohm is a parsing toolkit consisting of a library and a domain-specific language. You can use it to parse custom file formats or quickly build parsers, interpreters, and compilers for programming languages.

The Ohm language is based on parsing expression grammars (PEGs), which are a formal way of describing syntax, similar to regular expressions and context-free grammars. The Ohm library provides a JavaScript interface for creating parsers, interpreters, and more from the grammars you write.

• Full support for left-recursive rules means that you can define left-associative operators in a natural way.
• Object-oriented grammar extension makes it easy to extend an existing language with new syntax.
• Modular semantic actions. Unlike many similar tools, Ohm completely separates grammars from semantic actions. This separation improves modularity and extensibility, and makes both grammars and semantic actions easier to read and understand.
• Online editor and visualizer. The Ohm Editor provides instant feedback and an interactive visualization that makes the entire execution of the parser visible and tangible. It'll make you feel like you have superpowers. 💪

Some awesome things people have built using Ohm:

• Seymour, a live programming environment for the classroom.
• Chorus, a project exploring the middle ground between spreadsheets and programming.
• Shadama, a particle simulation language designed for high-school science.
• turtle.audio, an audio environment where simple text commands generate lines that can play music.
• A browser-based tool that turns written Konnakkol (a South Indian vocal percussion art) into audio.
• Wildcard, a browser extension that empowers anyone to modify websites to meet their own specific needs, uses Ohm for its spreadsheet formulas.

Getting Started

The easiest way to get started with Ohm is to use the interactive editor. Alternatively, you can play with one of the following examples on JSFiddle:

• Basic parsing example
• Arithmetic example with semantics

Resources

• Tutorial: Ohm: Parsing Made Easy
• The math example is extensively commented and is a good way to dive deeper.
• Examples
• Documentation

Installation

For use in the browser:

• Download ohm.js (development version, with full source and comments) or ohm.min.js (a minified version for faster page loads).

• Add a new script tag to your page, and set the src attribute to the path of the file you just downloaded. E.g.:

  <script src="ohm.js"></script>

  This creates a global variable named ohm.

If you are using Node.js, you can just install the ohm-js package using npm:

npm install ohm-js

This will install Ohm in the local node_modules folder. Use require to access it from a Node script:

const ohm = require('ohm-js');

Basics

Defining Grammars

To use Ohm, you need a grammar that is written in the Ohm language. The grammar provides a formal definition of the language or data format that you want to parse. There are a few different ways you can define an Ohm grammar:

• Define the grammar directly in a JavaScript string and instantiate it using ohm.grammar():

  const myGrammar = ohm.grammar('MyGrammar { greeting = "Hello" | "Hola" }');

  This is the simplest option, but it can be awkward to define larger grammars this way.

• Recommended when running in the browser: Embed the grammar source inside its own <script> tag with the attribute type="text/ohm-js", and instantiate it using ohm.grammarFromScriptElement():

  <script type="text/ohm-js">
    MyGrammar {
      greeting = "Hello" | "Hola"
    }
  </script>
  <script>
    const myGrammar = ohm.grammarFromScriptElement();
  </script>

• Recommended with Node.js: Define the grammar in a separate file, read the file's contents and instantiate it using ohm.grammar(contents):

  In myGrammar.ohm:

    MyGrammar {
      greeting = "Hello" | "Hola"
    }
  

  In JavaScript:

  const fs = require('fs');
  const ohm = require('ohm-js');
  const contents = fs.readFileSync('myGrammar.ohm');
  const myGrammar = ohm.grammar(contents);

For more information, see Instantiating Grammars in the API reference.

Using Grammars

Once you've instantiated a grammar object, use the grammar's match() method to recognize input:

const userInput = 'Hello';
const m = myGrammar.match(userInput);
if (m.succeeded()) {
  console.log('Greetings, human.');
} else {
  console.log("That's not a greeting!");
}

The result is a MatchResult object. You can use the succeeded() and failed() methods to see whether the input was recognized or not.

For more information, see the main documentation.

Debugging

Ohm has two tools to help you debug grammars: a text trace, and a graphical visualizer.

You can try the visualizer online, or if you have an Ohm checkout, open visualizer/index.html in your web browser.

To see the text trace for a grammar g, just use the g.trace() method instead of g.match. It takes the same arguments, but instead of returning a MatchResult object, it returns a Trace object — calling its toString method returns a string describing all of the decisions the parser made when trying to match the input. For example, here is the result of g.trace('ab').toString() for the grammar G { start = letter+ }:

ab         ✓ start ⇒  "ab"
ab           ✓ letter+ ⇒  "ab"
ab             ✓ letter ⇒  "a"
ab                 ✓ lower ⇒  "a"
ab                   ✓ Unicode [Ll] character ⇒  "a"
b              ✓ letter ⇒  "b"
b                  ✓ lower ⇒  "b"
b                    ✓ Unicode [Ll] character ⇒  "b"
               ✗ letter
                   ✗ lower
                     ✗ Unicode [Ll] character
                   ✗ upper
                     ✗ Unicode [Lu] character
                   ✗ unicodeLtmo
                     ✗ Unicode [Ltmo] character
           ✓ end ⇒  ""

Publishing Grammars

If you've written an Ohm grammar that you'd like to share with others, see our suggestions for publishing grammars.

Contributing to Ohm

All you need to get started:

git clone https://github.com/harc/ohm.git
cd ohm
npm install

NOTE: We recommend using the latest Node.js stable release.

Some useful scripts

• npm test runs the unit tests.
• npm run test-watch re-runs the unit tests every time a file changes.
• npm run build builds dist/ohm.js and dist/ohm.min.js, which are stand-alone bundles that can be included in a webpage.
• When editing Ohm's own grammar (in src/ohm-grammar.ohm), run npm run bootstrap to re-build Ohm and test your changes.

Before submitting a pull request, be sure to add tests, and ensure that npm run prepublish runs without errors.