Partial Application Syntax for ECMAScript

This proposal introduces syntax for a new calling convention (using ~()) to allow you to partially apply an argument list to a call or new expression through the combination of applied arguments (actual values) and placeholder arguments (unbound arguments that become parameters in the resulting partially applied function).

Status

Stage: 1
Champion: Ron Buckton (@rbuckton)

For more information see the TC39 proposal process.

Authors

• Ron Buckton (@rbuckton)

Proposal

Partial function application allows you to fix a number of arguments to a function call, returning a new function. Partial application is supported after a fashion in ECMAScript today through the use of either Function.prototype.bind or arrow functions:

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

// Function.prototype.bind
const addOne = add.bind(null, 1);
addOne(2); // 3

// arrow functions
const addTen = x => add(x, 10);
addTen(2); // 12

However, there are several of limitations with these approaches:

• Function.prototype.bind can only fix the leading arguments of a function.
• Function.prototype.bind requires you explicitly specify the this receiver.
• Arrow functions lazily re-evaluate their bodies, which can introduce unintended side-effects.

To resolve these concerns, we propose the introduction of the following new language features:

• A new calling convention, ~(), to indicate a partial application of a call or new expression which results in a partially applied function.
• Using the ? token to act as a placeholder argument for any non-fixed argument in a partial application.
• Using the ? token followed by a decimal integer literal (i.e., ?0) to act as an ordinal placeholder argument for non-fixed arguments bound to a specific ordinal parameter in the resulting partially applied function.
• Using the ... token to act as a rest placeholder argument for any excess arguments.

const add = (x, y) => x + y;
const identity = x => x;

// apply from the left:
const addOne = add~(1, ?);
addOne(2); // 3

// apply from the right:
const addTen = add~(?, 10);
addTen(2); // 12

// accept a fixed argument list:
const numbers = ["1", "2", "3"].map(parseInt~(?, 10)); // [1, 2, 3]

// specify ordinal placeholder arguments:
const indices = [1, 2, 3].map(identity~(?1)); // [0, 1, 2]

// bind `console` as receiver and accepts exactly one argument:
[1, 2, 3].forEach(console.log~(?));
// prints:
// 1
// 2
// 3

// emulate n-ary arguments like Function.prototype.bind:
const logger = console.log~("[service]", ...);
logger("foo", "bar"); // prints: [service] foo bar

Syntax

The ~() Partial Application Calling Convention

A partially applied call uses a separate calling convention than a normal call. Instead of using () to call or construct a value, you initiate a partial call using ~(). A partially applied call without a placeholder argument essentially fixes any provided arguments into a new function. If the expression being invoked produces a Reference, the this binding of the Reference is preserved. Excess arguments supplied to the resulting function are ignored by default (for more information, see Fixed Arity and Variable Arity later on in this document).

const sayNothing = console.log~();
const sayHi = console.log~("Hello!");

sayNothing();       // prints:
sayNothing("Shhh"); // prints:

sayHi();            // prints: Hello!

const bob = {
  name: "Bob",
  introduce() {
    console.log(`Hello, my name is ${this.name}.`);
  }
};

const introduceBob = bob.introduce~();
introduceBob();     // prints: Hello, my name is Bob.

This would not be the first new calling convention in ECMAScript, which also has tagged templates (i.e., tag`text${expr}`) and nullish function evaluation (i.e., f?.()).

The ? Placeholder Argument

The ? Placeholder Argument can be supplied one or more times at the top level of the argument list of a call or new expression (e.g. f~(?) or o.f~(?)). ? is not an expression, rather it is a syntactic element that indicates special behavior (much like how `...` AssignmentExpression indicates spread, yet is itself not an expression).

// valid
f~(x, ?)          // partial application from left
f~(?, x)          // partial application from right
f~(?, x, ?)       // partial application for any arg
o.f~(x, ?)        // partial application from left
o.f~(?, x)        // partial application from right
o.f~(?, x, ?)     // partial application for any arg
super.f~(?)       // partial application allowed for call on |SuperProperty|
new C~(?)         // partial application of constructor

// invalid
f~(x + ?)         // `?` not in top-level Arguments of call
x + ?             // `?` not in top-level Arguments of call
?.f~()            // `?` not in top-level Arguments of call
super~(?)         // `?` not supported in |SuperCall|
import~(?)        // `?` not supported in |ImportCall|

The ?0 (?1, ?2, etc.) Ordinal Placeholder Argument

The ? token can be followed by a decimal integer value ≥ 0 indicating a fixed ordinal position (i.e., ?0) denoting an Ordinal Placeholder Argument. Ordinal placeholder arguments are especially useful for adapting existing functions to be used as callbacks to other functions expect arguments in a different order:

const printAB = (a, b) => console.log(`${a}, ${b}`);
const acceptBA = (cb) => cb("b", "a");
acceptBA(printAB~(?1, ?0));                // prints: a, b

In addition, ordinal placeholder arguments can be repeated multiple times within a partial application, allowing repeated references to the same argument value:

const add = (x, y) => x + y;
const dup = add(?0, ?0);
console.log(dup(3));                       // prints: 6

Non-ordinal placeholder arguments are implicitly ordered sequentially from left to right. This means that an expression like f~(?, ?) is essentially equivalent to f~(?0, ?1). If a partial application contains a mix of ordinal placeholder arguments and non-ordinal placeholder arguments, ordinal placeholder arguments do not affect the implicit order assigned to non-ordinal placeholder arguments:

const printABC = (a = "arg0", b = "arg1", c = "arg2") => console.log(`${a}, ${b}, ${c}`);
printABC(1, 2, 3);                         // prints: 1, 2, 3
printABC();                                // prints: arg0, arg1, arg2

const printCAA = printABC~(?2, ?, ?0);     // equivalent to: printABC~(?2, ?0, ?0)
printCAA(1, 2, 3);                         // prints: 3, 1, 1
printCAA(1, 2);                            // prints: arg0, 1, 1

const printCxx = printABC~(?2);
printCxx(1, 2, 3);                         // prints: 3, arg1, arg2

By having ordinal placeholder arguments independent of the ordering for non-ordinal placeholder arguments, we avoid refactoring hazards due to the insertion a new ordinal placeholder into an existing partial application.

inserting an ordinal placeholder as the first argument:

-  const g = f~(?, ?, ?);                   // equivalent to: f~(?0, ?1, ?2)
+  const g = f~(?2, ?, ?, ?);               // equivalent to: f~(?2, ?0, ?1, ?2)

inserting an ordinal placeholder between other placeholders:

-  const g = f~(?, ?, ?);                   // equivalent to: f~(?0, ?1, ?2)
+  const g = f~(?, ?, ?0, ?);               // equivalent to: f~(?0, ?1, ?0, ?2)

Fixed Arity

By default, partial application uses a fixed argument list: Normal arguments are evaluated and bound to their respective argument position, and placeholder arguments (?) and ordinal-placeholder arguments (?0, etc.) are bound to specific argument positions in the resulting partially applied function. As a result, excess arguments passed to a partially applied function have no specific position in which they should be inserted. While this behavior differs from f.bind(), a fixed argument list allows us to avoid unintentionally accepting excess arguments:

// (a)
[1, 2, 3].forEach(console.log.bind(console, "element:"));
// prints:
// element: 1 0 1,2,3
// element: 2 1 1,2,3
// element: 3 2 1,2,3

// (b)
[1, 2, 3].forEach(x => console.log("element:", x));
// prints:
// element: 1
// element: 2
// element: 3

// (c)
[1, 2, 3].forEach(console.log~("element:", ?));
// prints:
// element: 1
// element: 2
// element: 3

In the example above, (a) prints extraneous information due to the fact that forEach not only passes the value of each element as an argument, but also the index of the element and the array in which the element is contained.

In the case of (b), the arrow function has a fixed arity. No matter how many excess arguments are passed to the callback, only the x parameter is forwarded onto the call.

The intention of partial application is to emulate a normal call like console.log("element:", 1), where evaluation of the "applied" portions occurs eagerly with only the placeholder arguments being "unapplied". This means that excess arguments have no place to go as part of evaluation. As a result, (c) behaves similar to (b) in that only a single argument is accepted by the partial function application and passed through to console.log.

Variable Arity: Pass Through Remaining Arguments using ...

However, sometimes you may need the variable arity provided by Function.prototype.bind. To support this, partial application includes a ... rest placeholder argument with a specific meaning: Take the rest of the arguments supplied to the partial function and spread them into this position:

const writeLog = (header, ...args) => console.log(header, ...args);
const writeAppLog = writeLog~("[app]", ...);
writeAppLog("Hello", "World!");
// prints:
// [app] Hello World!

const writeAppLogWithBreak = writeAppLog~(..., "\n---");
writeAppLogWithBreak("End of section");
// prints:
// [app] End of section
// ---

A partial application may only have a single ... rest placeholder argument in its argument list, though it may spread in other values using ...expr as you might in a normal call:

const arr = [1, 2, 3];

// The following would be a SyntaxError as the `...` placeholder may only appear once:
// const g = console.log~(?, ..., ...);

// However, a normal spread is perfectly valid. Below, `...arr` will be evaluated immediately
// and spread into the list of applied arguments:
const g = console.log~(?, ...arr, ...);
g("a", "b", "c");                           // prints: a, 1, 2, 3, b, c

Semantics

A call or new expression that uses the ~() calling convention results in a partially applied function. This result is a new function with a parameter for each placeholder argument (i.e., ?, ?0, etc.) in the argument list. If the partial application contains a ... rest placeholder argument, a rest parameter is added as the final parameter of the resulting partially applied function. Any non-placeholder arguments in the argument list becomes fixed in their positions. This is illustrated by the following syntactic conversion:

const g = f~(?, 1, ?);

is roughly identical in its behavior to:

const g = (() => {
  // applied values
  const _callee = f;
  const _applied0 = 1;

  // partially applied function
  return function (_0, _1) { return _callee(_0, _applied0, _1); };
})();

In addition to fixing the callee and any applied arguments, we also fix the the this receiver in the resulting partially applied function. As such, o.f~(?) will maintain o as the this receiver when calling o.f. This can be illustrated by the following syntactic conversion:

const g = o.f~(?, 1);

is roughly identical in its behavior to:

const g = (() => {
  // applied values
  const _receiver_ = o;
  const _callee = _receiver_.f;
  const _applied0 = 1;

  // partially applied function
  return function (_0) { return _callee.call(_receiver_, _0, _applied0); };
})();

The following is a list of additional semantic rules:

• Given f~(), the expression f is evaluated immediately, returning a partially applied function that always calls the value of f with no parameters.
• Given f~(?), the expression f is evaluated immediately, returning a partially applied function with a single parameter that always calls the value of f with that parameter as its sole argument.
• Given f~(?, x), the non-placeholder argument x is evaluated immediately and fixed in its position.
• Given f~(?), excess arguments supplied to the partially applied function result are ignored.
• Given f~(?, ?) the partially applied function result will have a parameter for each placeholder token that is supplied in that token's position in the argument list.
• Given f~(this, ?), the this in the argument list is the lexical this.
• Given f~(?), the this receiver of the function f is fixed as undefined in the partially applied function result.
• Given f~(?), the length of the partially applied function result is equal to the number of ? placeholder tokens in the argument list.
• Given f~(?), the name of the partially applied function result is f.name.
• Given o.f~(?), the references to o and o.f are evaluated immediately.
• Given o.f~(?), the this receiver of the function o.f is fixed as o in the partially applied function result.
• Given new C~(), the result is a function that returns a new instance of C.
  • NOTE: This is not easily achievable with .bind() today (if at all).
• Given const g = new C~(), g.name is "bound C".
• Given const g = new C~(), g.length is 0 (based on length derived from placeholder arguments).
• Given const g = new C~(), Object.getPrototypeOf(g) is C.
• Given new (f~()), the partial application of f returns a new function that can be constructed via new, similar to new (f.bind(null)).
• Given f?.~() (a partially applied, optional call), if f is null or undefined, the result is undefined. Otherwise, the result is the partial application of f~().
• Given o?.f~() (a partially applied call in an optional chain), if o is null or undefined, the result is undefined. Otherwise, the result is the partial application of o.f~().

Parsing

While this proposal leverages the existing ? token used in conditional expressions, it does not introduce parsing ambiguity as the ? placeholder token can only be used in an argument list and cannot have an expression immediately preceding it (e.g. f~(a? is definitely a conditional while f~(? is definitely a placeholder).

Grammar

MemberExpression[Yield, Await] :
  ...
  `new` MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await, ~Partial]

CallExpression[Yield, Await] :
  CallExpression[?Yield, ?Await] Arguments[?Yield, ?Await, +Partial]

CoverCallExpressionAndAsyncArrowHead[Yield, Await]:
  MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await, +Partial]

CallMemberExpression[Yield, Await] :
  MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await, +Partial]

SuperCall[Yield, Await] :
  `super` Arguments[?Yield, ?Await, ~Partial]

OptionalChain[Yield, Await] :
  `?.` Arguments[?Yield, ?Await, +Partial]
  ...
  OptionalChain[?Yield, ?Await] Arguments[?Yield, ?Await, +Partial]
  ...

Arguments[Yield, Await, Partial] :
  `(` ArgumentList[?Yield, ?Await, ~Partial] `)`
  `(` ArgumentList[?Yield, ?Await, ~Partial], `,` `)`
  [+Partial] [no LineTerminator here] `~` `(` ArgumentList[?Yield, ?Await, +Partial] `)`
  [+Partial] [no LineTerminator here] `~` `(` ArgumentList[?Yield, ?Await, +Partial] `,` `)`

ArgumentList[Yield, Await, Partial] :
  AssignmentExpression[+In, ?Yield, ?Await]
  `...` AssignmentExpression[+In, ?Yield, ?Await]
  ArgumentList[?Yield, ?Await, ?Partial] `,` AssignmentExpression[+In, ?Yield, ?Await]
  ArgumentList[?Yield, ?Await, ?Partial] `,` `...` AssignmentExpression[+In, ?Yield, ?Await]
  [+Partial] `?` DecimalIntegerLiteral?
  [+Partial] `...`
  [+Partial] ArgumentList[?Yield, ?Await, ?Partial] `,` `?` DecimalIntegerLiteral?
  [+Partial] ArgumentList[?Yield, ?Await, ?Partial] `,` `...`

NOTE: It is a SyntaxError for a partial call to have more than one ... placeholder.

Examples

Logging with Timestamps

const log = console.log~({ toString() { return `[${new Date().toISOString()}]` } }, ?);
log("test"); // [2018-07-17T23:25:36.984Z] test

Event Handlers

button.addEventListener("click", this.onClick~(?));

Bound methods

class Collator {
  constructor() {
    this.compare = this.compare~(?, ?);
  }
  compare(a, b) { ... }
}

Passing state through callbacks

// doWork expects a callback of `(err, value) => void`
function doWork(callback) { ... }
function onWorkCompleted(err, value, state) { ... }
doWork(onWorkCompleted~(?, ?, { key: "value" }));

Uncurry this

const slice = Array.prototype.slice.call~(?, ?, ?);
slice({ 0: "a", 1: "b", length: 2 }, 1, 2); // ["b"]

You can also find a number of desugaring examples in EXAMPLES.md.

Relationships to Other Proposals/Language Features

Partial Application and Pipeline

The Pipeline Proposal recently advanced to Stage 2 using the Hack-style for pipelines. While partial application was intended to dovetail with F#-style pipelines, this recent change does not diminish the value of partial application. In fact, the move to Hack-style mitigates the requirement that partial application not have a prefix token, which was a blocking concern from some members of TC39. That said, there is still a place for partial application in conjunction with pipeline:

const add = (x, y) => x + y;
const greaterThan = (x, y) => x > y;

// using Hack-style pipes
elements
  |> map(^, add~(?, 1))
  |> filter(^, greaterThan~(?, 5));

This creates a visual distinction between the topic variable in a Hack-style pipe (^ currently, although that has not been finalized), a partial call (~()), and a placeholder argument (?) that should aid in readability and improve developer intuition about their code will evaluate.

Partial Application and Optional Chaining

Partial Application is supported within an OptionalChain, per the Semantics and Grammar sections, above. As partial application is tied to Arguments, the ~( calling convention would follow ?. in an optional call:

const maybeAddOne = add?.~(?, 1); // undefined | Function
const maybeLog = console?.log~(?); // undefined | Function

Per the semantics of OptionalChain, in both of the examples above the ?. token short-circuits evaluation of the rest of the chain. As a result, if the callee is nullish then the result of both expressions would be undefined. If the callee is not nullish, then the result would be the partial application of the callee.

Open Questions/Concerns

Choosing a different token than ?

There have been suggestions to consider another token aside from ?, given that optional chaining may be using ?. and nullish coalesce may be using ??. It is our opinion that such a token change is unnecessary, as ? may only be used on its own in an argument list and may not be combined with these operators (e.g. f~(??.a ?? c) is not legal). The ? token's visual meaning best aligns with this proposal, and its fairly easy to write similarly complex expressions today using existing tokens (e.g. f(+i+++j-i---j) or f([[][]][[]])). A valid, clean example of both partial application, optional chaining, and nullish coalesce is not actually difficult to read in most cases: f~(?, a?.b ?? c).

Definitions

• Partial Application — A call or new expression with zero or more placeholder arguments, where applied expressions are immediately evaluated and fixed in their respective positions in the invocation.
  A partial application is denoted by an argument list surrounded by ~()
  Example: f~()
• Partially Applied Function — A function that is the result of a partial application.
• Applied Expressions — The callee, receiver, and any non-placeholder arguments of a partial application.
  Also: applied.
  Antonyms: unapplied.
• Callee — The value of the function or method to be invoked (for a call) or instantiated (for new expressions).
• Receiver — If the Callee is a method invocation, the receiver is the object that will be passed to the call as its this binding.
• Fix — Eagerly evaluate an expression and store its position within a partially application.
  Also: fixed, fixing.
• Non-Placeholder Argument — An Applied Expression that takes up an entire argument position.
• Placeholder Argument — An argument that is not yet applied in a partial application. A placeholder argument results in one or more parameter bindings in a resulting partially applied function.
• Non-Ordinal Placeholder Argument — A placeholder argument representing a single unapplied argument. Non-ordinal placeholder arguments are implicitly ordered sequentially from left to right.
  A non-ordinal placeholder argument is denoted by a ? token that takes up an entire an argument position.
  Example: f~(?)
• Ordinal Placeholder Argument — A placeholder argument representing a single unapplied argument with a specified ordinal position in the parameter list of the resulting partially applied function.
  An ordinal placeholder argument is denoted by a ? token followed by an unsigned integer indicating the ordinal position of the resulting parameter.
  Example: f~(?1, ?0)
• Rest Placeholder Argument — A placeholder argument representing any excess arguments passed to the resulting partially applied function.
  A rest placeholder argument is denoted by a ... token that takes up an entire argument position.
  Example: f~(...)

Resources

• Overview Slides

TODO

The following is a high-level list of tasks to progress through each stage of the TC39 proposal process:

Stage 1 Entrance Criteria

• [x] Identified a "champion" who will advance the addition.
• xProse outlining the problem or need and the general shape of a solution.
• [x] Illustrative examples of usage.
• [x] ~High-level API~ (proposal does not introduce an API).

Stage 2 Entrance Criteria

• [x] Initial specification text.
• [ ] Optional. Transpiler support.

Stage 3 Entrance Criteria

• [ ] Complete specification text.
• [ ] Designated reviewers have signed off on the current spec text.
• [ ] The ECMAScript editor has signed off on the current spec text.

Stage 4 Entrance Criteria

• [ ] Test262 acceptance tests have been written for mainline usage scenarios and merged.
• [ ] Two compatible implementations which pass the acceptance tests: [1], [2].
• [ ] A pull request has been sent to tc39/ecma262 with the integrated spec text.
• [ ] The ECMAScript editor has signed off on the pull request.