ECMAScript Proposal: Function.prototype.papp

This proposal introduces papp and pappRight – concise ways of using partial application for functions that require no immediate this parameter. It is backwards-compatible, and is immediately useful with most any JavaScript function today.

Try it out!

npm install --save papp-polyfill

Then require it once (in your code, as early as possible):

require('papp-polyfill')

Introduction

Partial application is possible in JavaScript via Function.prototype.bind:

function add (x, y) { return x + y; }

var addTen = add.bind(null, 10);
addTen(20) //=> 30

However, bind is undesirable for three reasons:

1. Sometimes you don't care about the value of this, yet you still must provide bind's first argument
2. Sometimes you do care about the value of this, but don't want to commit to a specific value yet.
3. ~~Using bind is significantly slower than using a plain closure (!)~~ (this has been fixed in V8, and wasn't an issue in other engines for quite a while)

Function.prototype.papp solves both these issues in a simple, elegant, and noise-free manner. Here is an illustrative example:

function add (x, y, z) { return x + y + z; }

var addTen = add.papp(3, 7);
addTen(5) //=> 15

// AS OPPOSED TO:
// var addTen = add.bind(null, 3, 7)
// OR:
// var addTen = (x) => add(3, 7, x)

var addThenIncrement = add.papp(1);
addThenIncrement(10, 6) //=> 17

// AS OPPOSED TO:
// var addThenIncrement = add.bind(null, 1)
// OR:
// var addThenIncrement = (a, b) => add(1, a, b)

Accepting papp into the ES standard will allow JS runtimes to implement a more performant version of bind that is dedicated to partial application.

Ignoring this

For functions that don't use the keyword this, papp helps with brevity:

function add (x, y) { return x + y; }

var addTen = add.papp(10);
addTen(20) //=> 30

Deferring Function Binding

If a function does use the keyword this, papp allows you to partially apply arguments without committing to a this value:

function greet (target) {
  return `${this.name} greets ${target}`;
}

var greetNewcomer = greet.papp('the newcomer');
greetNewcomer.call({ name: 'Alice' }) //=> 'Alice greets the newcomer'

Practical Examples

These examples are pulled from real-world use cases of partial application.

HTTP API Output Whitelisting

Player.whitelist = {
  basic: pluck.papp(['name', 'score']),
  admin: pluck.papp(['name', 'score', 'email']),
};

function pluck (attrs, obj) {
  var result = {};
  attrs.forEach( name => result[name] = obj[name] );
  return result;
}

// Example use (in practice, alice would come from a database):
var alice = { name: 'Alice', score: 100, email: '[email protected]', password_hash: '...' };

Player.whitelist.basic(alice) //=> { name: 'Alice', score: 100 }

Player.whitelist.admin(alice) //=> { name: 'Alice', score: 100, email: '[email protected]' }

Constructing User-friendly APIs

function createClient (host) {
  return {
    get:  makeRequest.papp(host, 'GET'),
    post: makeRequest.papp(host, 'POST'),
    put:  makeRequest.papp(host, 'PUT'),
    del:  makeRequest.papp(host, 'DELETE'),
  };
}

var client = createClient('https://api.example.com');
client.get('/users');
client.post('/comments', { content: "papp is great!" });

function makeRequest (host, method, url, data, options) {
  // ... Make request, return a promise ...
}

// AS OPPOSED TO:
// function createClient (host) {
//   return {
//     get:  (url, data, options) => makeRequest(host, 'GET', url, data, options),
//     post: (url, data, options) => makeRequest(host, 'POST', url, data, options),
//     put:  (url, data, options) => makeRequest(host, 'PUT', url, data, options),
//     del:  (url, data, options) => makeRequest(host, 'DELETE', url, data, options),
//   }
// }

Other Examples

These examples illustrate concepts you can use in your own applications.

Mapping with Arrays

var chapters = ["The Beginning", "Climax", "Resolution"];

var numberedChapters = chapters.map( toListItem.papp('My Book') )
//=> ["My Book / 1. The Beginning", "My Book / 2. Climax", "My Book / 3. Resolution"]

// AS OPPOSED TO:
// var numberedChapters = chapters.map( (chapter, i) => toListItem('My Book', chapter, i) )

function toListItem (prefix, item, i) {
  return `${prefix} / ${i + 1}. ${item}`
}

Polyfill

ES6:

Function.prototype.papp = function (...args) {
  var fn = this;
  return function (...moreArgs) {
    fn.apply(this, args.concat(moreArgs));
  };
};

ES5:

Function.prototype.papp = function () {
  var slice = Array.prototype.slice;
  var fn = this;
  var args = slice.call(arguments);
  return function () {
    return fn.apply(this, args.concat(slice.call(arguments)));
  };
};