partial-strawman.md

Motivation

• Partial application is fundamental to functional-style programming
• It is the binding of arguments to parameters before a function is invoked
• This technique is often used by ECMAScript developers who are:
  • seeking to avoid the use of new and/or this
  • deliberately trying to avoid function genericism
  • trying to improve consistency of style
• Partial application is currently supported by Function.prototype.bind, but the syntax is verbose, and the target of the bound function comes first, diluting the semantic
• Writing a custom function to provide partial application is straightforward (indeed there are many open source libraries providing this such as _.partial), however developers taking this approach are constrained in their ability to achieve syntactic tenseness lest they surprise other developers
• A terse, native syntax for partial application that does not affect the target of a function will improve consistency, clarity of intent and legibility

Proposal

Introduce two partial-application operators :>: (partially apply left-to-right) and :<: (partially apply right-to-left). These operators create a bound function such that the arguments to be supplied to the function on the left-hand side of the operator, when it is eventually invoked, are bound to those values supplied on the right-hand side of the operator.

Before

import _ from 'lodash';
const o = { 
  foo: _.partial(foo, arg1),
  bar: bar.bind(null, arg1, arg2)
};

After

const o = { 
  foo:>:arg1,
  bar:>:[arg1, arg2]
};

Before

Promise.resolve()
  .then(() => foo(arg1));

After

Promise.resolve()
  .then(foo:>:arg1);

Examples

const arg1 = 'this is arg1';
function foo(arg1) { console.log(arg1); }

// Current syntax 1
o = { foo: foo.bind(null, arg1) };

// Current syntax 2
let o = { foo: _.partial(foo, arg1) };

// Current syntax 3
let o = { foo: (...args) => foo(arg1, ...args) };

// Proposed syntax
o = { foo:>:arg1 };

o.foo(); // 'this is arg1' printed to console

Use as part of an assignment expression:

const bound = foo:>:context;

As part of a return statement with the spread operator:

function curry(foo, ...args) { return foo:>:[...args]; };

Partial application with multiple arguments. The right-hand side value may be either a single value or an array of arguments:

var bound = foo:>:['a','b'];

If the single value needs to be an array we need to disambiguate:

var bound = foo:>:(['a','b']);

Partial application from the right:

const foo = (a, b) => console.log(a, b);
var bound = foo:<:['this is b'];
bound('this is a'); // 'this is a, this is b' printed to  console

Further considerations

• I am an unsophisticate with respect to syntax ambiguity avoidance
• The proposed syntax is merely a first guess and I am confident there will be better ideas
• I originally chose the syntax :: for the operator, but there is another stage 0 proposal that intends to use that syntax for a different purpose. Also the revised syntax permits identification of the binding direction (from left/from right)

Some comments to help improve the proposal:

• Why not a Function.prototype.partial-like proposal?
• How to write _.partial(foo, [arg1, arg2]) with this proposal?
• Would obj.func:>:arg1 be like obj.func.bind(null, arg1) or obj.func.bind(obj, arg1)?
• When is arg1 evaluated in the examples?

About my last point, the two following cases have different behavior, which behavior will this new operator follow?

let arg1 = 'this is arg1';
function foo(arg) { console.log(arg); }

const o1 = { foo: foo.bind(undefined, arg1) };
const o2 = { foo: (...args) => foo(arg1, ...args) };

arg1 = 'this is arg1 bis';

o1.foo();
o2.foo();

The proposal that is currently gaining most traction is here.