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 thethis
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 ornew
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
~()
Partial Application Calling Convention
The 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?.()
).
?
Placeholder Argument
The 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|
?0
(?1
, ?2
, etc.) Ordinal Placeholder Argument
The 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 expressionf
is evaluated immediately, returning a partially applied function that always calls the value off
with no parameters. - Given
f~(?)
, the expressionf
is evaluated immediately, returning a partially applied function with a single parameter that always calls the value off
with that parameter as its sole argument. - Given
f~(?, x)
, the non-placeholder argumentx
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, ?)
, thethis
in the argument list is the lexicalthis
. - Given
f~(?)
, thethis
receiver of the functionf
is fixed asundefined
in the partially applied function result. - Given
f~(?)
, thelength
of the partially applied function result is equal to the number of?
placeholder tokens in the argument list. - Given
f~(?)
, thename
of the partially applied function result isf.name
. - Given
o.f~(?)
, the references too
ando.f
are evaluated immediately. - Given
o.f~(?)
, thethis
receiver of the functiono.f
is fixed aso
in the partially applied function result. - Given
new C~()
, the result is a function that returns a new instance ofC
.- NOTE: This is not easily achievable with
.bind()
today (if at all).
- NOTE: This is not easily achievable with
- Given
const g = new C~()
,g.name
is"bound C"
. - Given
const g = new C~()
,g.length
is0
(based on length derived from placeholder arguments). - Given
const g = new C~()
,Object.getPrototypeOf(g)
isC
. - Given
new (f~())
, the partial application off
returns a new function that can be constructed vianew
, similar tonew (f.bind(null))
. - Given
f?.~()
(a partially applied, optional call), iff
isnull
orundefined
, the result isundefined
. Otherwise, the result is the partial application off~()
. - Given
o?.f~()
(a partially applied call in an optional chain), ifo
isnull
orundefined
, the result isundefined
. Otherwise, the result is the partial application ofo.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
TODO
The following is a high-level list of tasks to progress through each stage of the TC39 proposal process:
Stage 1 Entrance Criteria
- Identified a "champion" who will advance the addition.
- Prose outlining the problem or need and the general shape of a solution.
- Illustrative examples of usage.
-
High-level API(proposal does not introduce an API).
Stage 2 Entrance Criteria
- 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.