ELF - ECMAScript Language Framework

Introduction

ELF let's you create reusable and extensible programming language implementations in JavaScript. It implements abstractions for creating parsers and lexers with automatic error-recovery, easy handling of operator precedence and (basic) AST walkers with support for pattern-matching.

ELF takes advantage of JavaScript's prototypical nature so the way you create languages is by 'cloning' existing ones can in turn be cloned by more specific ones. This, combined with the ability to mixin and borrow rules, makes it very easy to extend and compose existing languages.

Installation

> mkdir my-lang
> cd my-lang
> npm install elf.js

Calculator Example

var elf = require('elf.js'), _;

var Calculator = elf.Language.clone(function () {
  this.number(/\d+/)
  
  this.prefix("-")
  
  this.infix("+", 10)
  this.infix("-", 10)
  this.infix("*", 20)
  this.infix("/", 20)
  
  this.stmt ("print", function (node) {
    node.first = this.expression();
    node.arity = "statement";
    return node;
  });
  
  this.skip(/\s+/)
  this.eol(";")
});

var Interpreter = elf.Walker.clone(function () {
  this.match('-', [ _ ], function (node, right) {
  	return -this.walk(right);
  })
  
  this.match('-', [ _ , _ ], function (node, left, right) {
  	return this.walk(left) - this.walk(right);
  })
  
  this.match('+', function (node, left, right) {
  	return this.walk(left) + this.walk(right);
  })
  
  this.match('*', function (node, left, right) {
  	return this.walk(left) * this.walk(right);
  })
  
  this.match('*', [ _ , 2 ], function (node, left, right) {
  	return this.walk(left) >> 1;
  })
  
  this.match('/', function (node, left, right) {
  	return this.walk(left) / this.walk(right);
  })
  
  this.match('print', function (node, first) {
  	console.log(this.walk(first));
  });
  
  this.match(_, function (node) {
  	return node.value;
  })
});

var ast = Calculator.parse('print 1 + 2 * 3;print 4-2;');

Interpreter.walk(ast);

API

elf.Object

Provides an abstraction over JavaScript's implementation of protoypal inheritence.

• .clone(initializer<function|object>)

  Returns a new object whose prototype link points to the object being cloned. It accepts either a function or an object with initialization logic which will be applied to the new object.

• .extend(…mixins)

  Copies all properties from each of the mixin objects over to itself.

• .slots()

  Returns an array of all it's properties.

elf.Token

Represents a lexical token.

• .create(type, value)

  Creates and returns a new token object from the current one with type and value set

• .error(message<string>)

  Turns the token into an error token

• .pos(pos<object>)

  Updates the token position

elf.Lexer

Provides a lexer abstraction on top of which you can base your own more specific lexers

• .name(match<regexp|string>, [action<function>])

  Creates a rule for matching identifiers based on the provided match object.

• .number(match<regexp|string>, [action<function>])

  Creates a rule for matching number literals based on the provided match object.

• .string(match<regexp|string>, [action<function>])

  Creates a rule for matching strings literals based on the provided match object.

• .regex(match<regexp|string>, [action<function>])

  Creates a rule for matching regex literals based on the provided match object.

• .operator(match<regexp|string>, [action<function>])

  Creates a rule for matching operators based on the provided match object.

• .eol(match<regexp|string>, [action<function>])

  Creates a rule for matching end-of-line operators based on the provided match object.

• .skip(match<regexp|string>)

  Tells the lexer to skip anything matching the provided match object.

• .rule(name, regex, action, arity)

  A lower-level matcher that all of the previous rules are based upon.

  All of the above methods accepts an optional action function that will be invoked when the rule matches. The context inside the action is the matched string. It can return one of three things: a value (which will be set as the value on the current token), an array of tokens or null (in which case the lexer will ignore the match).

elf.Parser

Provides a parser abstraction on top of which you can base your own more specific parsers

• .advance([id])

  Advances to the next token, if an id is provided an error node will be created if it doesn't match the id of that token.

• .expression([rbp])

  Parses a single expression, if an rbp (right binding power) is provided then it will only parse as long as the next token has the same binding power or higher.

• .statement()

  Parses a single statement.

• .block(openTag, closeTag)

  Parses a block of code delimited by openTag and closeTag.

• .stmt(id, std<function(node)>)

  Creates a rule for matching a statement. A statement can only appear at the beginning of an expression.

• .prefix(id, [nud<function(node)>])

  Creates a rule for matching prefix tokens, similar to stmt exept that it can appear multiple times in an expression.

• .infix(id, bp, [led<function(node, left)>])

  Creates a rule for matching tokens in infix position. Can appear multiple times in an expression.

• .infixr(id, bp, [led<function(node, left)>])

  Like infix except that it associates to the right.

• .borrow(parser, ...rules)

  Let's you borrow a set of rules from another parser.

• .token

  Always points to the current token.

• .tokens

  A collection of all the remaining tokens. It has a .peek([n]) method that let's you look at the next token without consuming it.

• .parse(tokens<Array>)

  Accepts a collection of tokens and returns an AST.

elf.Language

Provides a unified abstraction for lexers and parsers.

• .name(regex<regexp|string>, [action<function(token)>])

  Creates a rule for matching identifiers based on the provided match object.

• .number(regex<regexp|string>, [action<function(token)>])

  Creates a rule for matching numbers based on the provided match object.

• .string(regex<regexp|string>, [action<function(token)>])

  Creates a rule for matching strings based on the provided match object.

• .regex(regex<regexp|string>, [action<function(token)>])

  Creates a rule for matching regular expressions based on the provided match object.

• .eol(regex<regexp|string>, [action<function(token)>])

  Creates a rule for matching end-of-line operators based on the provided match object.

• .skip(regex<regexp|string>, [action<function(token)>])

  Tells the language to skip anything matching the provided match object.

• .stmt(id<string>, std<function(node)>);

  Creates a rule for matching a statement. A statement can only appear at the beginning of an expression.

• .prefix(id<string>, [nud<function(node)>])

  Creates a rule for matching prefix tokens, similar to stmt exept that it can appear multiple times in an expression.

• .infix(id<string>, bp<integer>, [led<function(node, left)>])

  Creates a rule for matching tokens in infix position. Can appear multiple times in an expression.

• .infixr(id<string>, bp<integer>, [led<function(node, left)>])

  Like infix except that it associates to the right.

• .borrow(language, ...rules)

  Let's you borrow a set of rules from another parser.

• .parse(input<string>)

  Accepts a program string and returns an AST.

elf.Walker

Recursively walks the AST and applies your matchers.

• .match(match<string|number>, [pattern<array>], action<function(node, child1, child2, …, childN)>)

  Executes the action if the type or value of the current node matches the match argument and if the provided (optional) pattern matches.

• .walk(ast<AST|Node|Array>)

  Walks the ast and returns whatever your actions return.

elf.Walker

Recursively walks the AST collecting error nodes.

• .walk(ast<AST|Node|Array>)

  Walks the ast and generates an error report.