ECMAScript Proposal: Function.prototype.papp
This proposal introduces papp and pappRight β 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) //=> 30However, bind is undesirable for three reasons:
- Sometimes you don't care about the value of
this, yet you still must providebind's first argument - Sometimes you do care about the value of
this, but don't want to commit to a specific value yet. Using(this has been fixed in V8, and wasn't an issue in other engines for quite a while)bindis significantly slower than using a plain closure (!)
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) //=> 30Deferring 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) {
return fn.apply(this, [...args, ...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)));
};
};