1. Ron Buckton – Microsoft Corporation
Partial Application
Ron Buckton – Microsoft Corporation

2. Motivations Partially supported via ‘Function#bind()’
Can only fix leading arguments.
Explicit specification of ‘this’ receiver.
All remaining arguments are passed to bound function.
Supported via arrow functions
With Pipeline, need to write |> _ => for each step.
With Pipeline, stack depth becomes ambiguous with multiple steps.
const add = (x, y) => x + y;
// Function#bind
const addOne = add.bind(null, 1);
addOne(2); // 3
// arrow functions
const addTen = x => add(x, 10);
addTen(2); // 12
// arrow functions and pipeline
const newScore = player.score
|> _ => add(7, _)
|> _ => clamp(0, 100, _);
Motivations

3. Allow partial application of function calls by fixing arguments and providing placeholders:
? placeholder token
… rest placeholder token
Placeholders only permitted in ArgumentList.
Result of call is a Function.
// `?` placeholder f(x, ?) // left apply
f(?, x) // right apply
f(?, x, ?) // middle apply
// `...` rest placeholder f(x, ...) // left apply
f(..., x) // right apply
f(..., x, ...) // middle apply // methods o.f(x, ?)
Proposal

4. // on its own const add = (x, y) => x + 1; const addOne = add(
// on its own const add = (x, y) => x + 1; const addOne = add(?, 1); [1, 2, 3].map(addOne); // [2, 3, 4] // argument limiting [1].forEach(console.log); // 1 0 [1] [1].forEach(console.log(?)); // 1 // uncurry 'this' const g = f.call(?, ...); // add listener el.addEventListener(this.clicked(?));
// with pipeline, underscore/lodash style const result = books |> filter(?, x => x.category === "programming"); // with pipeline, Ramda style const result = books |> filter(x => x.category === "programming", ?);
Examples

5. Open Questions Call-by-name vs. call-by-value?
Given const g = f(?), what happens when ‘f’ changes?
What about a.b()[c++](?)?
Eagerly evaluate fixed arguments?
Given const g = f(a, ?), what happens when ‘a’ changes?
Given const g = create({ a: 1 }, ?), should we get the same object literal value or a fresh one?
Open Questions

6. STATUS Stage: 0 Identified Champions: Ron Buckton, Brian Terlson
Strawman available
STATUS