Native pattern matching for JavaScript


Sparkler is a pattern matching engine for JavaScript built using sweet.js macros, so it looks and feels like native syntax. It has no runtime dependencies and compiles down to simple ifs and fors.

Here's a small slice of what you can do with it:

function myPatterns {
  // Match literals 
  42 => 'The meaning of life',
  // Tag checking for JS types using Object::toString 
  a @ String => 'Hello ' + a,
  // Array destructuring 
  [...front, back] => back.concat(front),
  // Object destructuring 
  { foo: 'bar', x, 'y' } => x,
  // Custom extractors 
  Email { user, domain: '' } => user,
  // Rest arguments 
  (a, b, => rest,
  // Rest patterns (mapping a pattern over many values) 
  [...{ x, y }] =>, y),
  // Guards 
  x @ Number if x > 10 => x

You can see a slew of more examples in the pattern spec file

npm install -g sweet.js
npm install sparkler
sjs -m sparkler/macros myfile.js

Sparkler overloads the function keyword as a macro (don't worry, all your old functions will still work) but implements a slightly different syntax. There's no argument list after the name or function keyword. Instead the function body is just a set of ES6 style arrow-lambdas separated by commas.

function myFunc {
  // Single arguments don't need parens and simple expression  
  // bodies get an implicit `return`. 
  a @ String => 'Hello ' + a,
  // Add parens and braces if you need more. 
  (x, y, z) => {
    return x + y + z;

You can also do this with anonymous functions:

DB.getResource('123', function {
  (null, resp) => complete(resp),
  (err) => handleError(err)

If no case matches, a TypeError('No match') is thrown.

Sparkler doesn't just try each case one at a time until one passes. That would be really inefficient. Instead, it analyzes your entire pattern matrix, and rearranges things as needed to get an optimized set of tests while still preserving the left-to-right, top-down semantics.

function expensiveExtraction {
  (MyExtractor(x, y, z), 1) => doThis(),
  (MyExtractor(x, y, z), *) => doThat()

Let's say MyExtractor is really expensive. Sparkler efficiently backtracks, so it will only get called once in this set of tests.

In JavaScript, you can call a function with any number of arguments. Arguments that are not provided are just set to undefined. Sparkler does not implicitly match on argument length.

function ambiguous {
  (a)       => 1,
  (a, b)    => 2,
  (a, b, c) => 3

The above function will always return 1 no matter how many arguments you call it with as the first case always matches. The subsequent cases are actually removed from the final output in the optimization phase.

If you want to match on specific argument length, you need to add a guard to your case.

function argCheck {
  // Using arguments.length 
  (a, b, c) if arguments.length == 3 => 1,
  // Or matching undefined 
  (a, b, undefined) => 2,
  (a) => 1

The only time Sparkler is strict with argument length is with the empty parameter list (). It will check that arguments.length is zero. This is so you can do stuff like this:

Foo.prototype = {
  // jQuery-style getter/setter 
  val: function {
    ()  => this._val,
    val => {
      this._val = val;
      return this;

If you want a catch-all, you should use a wildcard (*) or default instead.

Sparkler exports a match macro for doing easy matching in any position.

Match expressions look like function matching, except you provide the argument(s) upfront.

var num = 12;
var isNumber = match num {
  Number => true,
  *      => false

This works by desugaring match into a self-invoking function with num as the argument. Consequently, match expressions do not support break, continue, and early return. Using a match expression in statement position will result in a parse error.

Match statements use a slightly different syntax. They look like a suped up switch.

var a = Foo(Foo(Foo(42)));
while (1) {
  match a {
    case Foo(inner):
      a = inner;

Unlike switches, cases in a match statement do not fall through. Early return, break, and switch are all supported. Using a match statement in expression position will result in a parse error.

You can match on multiple expressions at once in both match expressions and statements.

var allNums = match (num1, num2, num3) {
  (Number, Number, Number) => true,
  *                        => false
match (num1, num2, num3) {
  case (Number, Number, Number):
    allNums = true;
    allNums = false;

All bindings in patterns are declared as vars by default, as it is the most widely supported declaration form. Consequently, they will hoist outside of match statements. You may specify your declaration form by prefixing pattern bindings with one of var, let, or const.

match x {
  case Foo(a):       ... // will hoist 
  case Foo(var a):   ... // will hoist 
  case Foo(let a):   ... // will not hoist 
  case Foo(const a): ... // will not hoist, immutable 

You can match on your own types by implementing a simple protocol. Let's build a simple extractor that parses emails from strings:

var Email = {
  // Factor out a matching function that we'll reuse. 
  match: function {
    x @ String => x.match(/(.+)@(.+)/),
    *          => null
  // `hasInstance` is called on bare extractors. 
  hasInstancefunction(x) {
    return !!Email.match(x);
  // `unapply` is called for array-like destructuring. 
  unapplyfunction(x) {
    var m = Email.match(x);
    if (m) {
      return [m[1], m[2]];
  // `unapplyObject` is for object-like destructuring. 
  unapplyObjectfunction(x) {
    var m = Email.match(x);
    if (m) {
      return {
        user: m[1],
        domain: m[2]

Now we can use it in case arguments:

function doStuffWithEmails {
  // Calls `unapplyObject` 
  Email { domain: '' } => ...,
  // Calls 'unapply' 
  Email('foo', *) => ...,
  // Calls `hasInstance` 
  Email => ...

If you don't implement hasInstance, Sparkler will fall back to a simple instanceof check.

adt-simple is a library that implements the extractor protocol out of the box, and even has its own set of macros for defining data-types.

var adt = require('adt-simple');
union Tree {
  Node {
    value : *,
    left  : Tree,
    right : Tree
} deriving (adt.Extractor)
function treeFn {
  Empty => 'empty',
  Node { value @ String } => 'string'

Sparkler also provides a small library that extends some of the native types with convenient functions.

// Date destructuring 
function dateStuff {
  Date { month, year } => ...
// RegExp destructuring 
function regexpStuff {
  RegExp { flags: { 'i' }} => ...
// Partial-function composition with `orElse` 
function partial {
  Foo => 'foo'
var total = partial.orElse(function {
  * => 'anything'

orElse is added to the prototype safely using defineProperty and isn't enumerable.

Nathan Faubion (@natefaubion)