| Pipelines | 321 |Temporary Variables | 322 | 323 | 324 | 325 |
|---|---|
| 327 | 328 | ```js 329 | const envVarFormat = vars => 330 | Object.keys(vars) 331 | .map(var => `${var}=${vars[var]}`) 332 | .join(' ') 333 | |> chalk.dim(%, 'node', args.join(' ')); 334 | ``` 335 | 336 | | 337 |338 | 339 | ```js 340 | const envVarFormat = (vars) => { 341 | let _ = Object.keys(vars); 342 | _ = _.map(var => `${var}=${vars[var]}`); 343 | _ = _.join(' '); 344 | return chalk.dim(_, 'node', args.join(' ')); 345 | } 346 | ``` 347 | 348 | | 349 |
352 |
353 | ```jsx
354 | // This example uses JSX.
355 | return (
356 |
|
372 |
373 |
374 | ```js
375 | // This example uses JSX.
376 | let _ = values;
377 | _= Object.keys(_);
378 | _= [...Array.from(new Set(_))];
379 | _= _.map(envar => (
380 |
|
390 |
% is not a final choice2 | title: ES pipe operator (2021) 3 | status: proposal 4 | stage: 2 5 | location: https://github.com/tc39/proposal-pipeline-operator 6 | copyright: false 7 | contributors: J. S. Choi, James DiGioia, Ron Buckton, Tab Atkins-Bittner 8 |9 | 10 | 11 | 12 |
This is the formal specification for a proposed Hack-style pipe 15 | operator `|>` in JavaScript. It modifies the original ECMAScript specification with 17 | several new or revised clauses. See the proposal's 19 | explainer for the proposal's background, motivation, and usage examples.
20 | 21 |This document presumptively uses `%` as the token 22 | for the topic reference. This choice of token is not a final decision; 23 | `%` could instead be `^` or some other token.
24 |This section augments the original 39 | IsFunctionDefinition clause.
40 | 41 |It presumptively uses `%` as the placeholder token for the 42 | topic reference. This choice of token is not a final decision; `%` 43 | could instead be `^` or some other token.
44 |This section augments the original IsIdentifierRef 72 | clause.
73 | 74 |It presumptively uses `%` as the placeholder token for the 75 | topic reference. This choice of token is not a final decision; `%` 76 | could instead be `^` or some other token.
77 |This section is a wholly new sub-clause of the original Contains clause.
114 |Several early error rules for |ScriptBody| and for 118 | |ModuleItemList|, as well as a step in CreateDynamicFunction, 119 | use the ContainsOuterTopic operation to check for any unbound topic reference `%`. 120 | Any inner topic reference within a |PipeBody| is hidden from these rules, 121 | preventing them from triggering the rules during program 122 | compilation.
123 | 124 |This guarantees that any topic reference in a program must be 125 | present within a topic-binding environment 126 | created by a |PipeBody| within that program.
127 |Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of ContainsOuterTopic:
130 | 131 |This section augments the original 163 | AssignmentTargetType clause.
164 | 165 |It presumptively uses `%` as the placeholder token for the 166 | topic reference. This choice of token is not a final decision; `%` 167 | could instead be `^` or some other token.
168 |This section augments the original 207 | Environment Records clause.
208 || Method | 215 |Purpose | 216 |
|---|---|
| Determine whether an Environment Record is a 224 | topic-binding environment. Return *true* if it establishes a 225 | topic binding and *false* if it does not. | 226 |
This section augments the original Declarative Environment Records clause.
238 | 239 |It presumptively uses `%` as the placeholder token for the 240 | topic reference. This choice of token is not a final decision; `%` 241 | could instead be `^` or some other token.
242 |Declarative Environment Records have the additional state fields
246 | listed in
| Method | 255 |Value | 256 |Purpose | 257 |
|---|---|---|
| [[TopicValues]] | 262 |List of any | 263 |If the declarative Environment Record is a topic-binding 264 | environment, then [[TopicValues]] is a List containing the one element 265 | which is the environment's topic value (that is, the value of 266 | the topic reference within its program scope). 267 | Otherwise, the value of [[TopicValues]] is an empty List. | 268 |
[[TopicValues]] is a List in order to be forward compatible with 275 | future extensions that would use more than one topic value, e.g., "pipe functions".
278 |Declarative Environment Records support all of the abstract methods
281 | of Environment Records listed in
| Method | 290 |Purpose | 291 |
|---|---|
| BindTopicValues(V) | 296 |Establish the immutable topic binding of this 297 | Environment Record and set the topic binding's value. 298 | _V_ is a List containing the one element which is the 299 | topic value and is a value of any ECMAScript language 300 | type. Afterward, the Environment Record is a 301 | topic-binding environment, and the value returned by the 302 | Environment Record's HasTopicBinding method is *true*. 303 | This method cannot be called more than once on any 304 | single Environment Record. | 305 |
BindTopicValues accepts a List argument rather than a single-value 312 | argument in order to be forward compatible with future extensions that 313 | would use more than one topic value, e.g., "pipe functions".
316 |The behaviour of the concrete and additional specification 320 | methods for declarative Environment Records is defined by the following 321 | algorithms.
322 | 323 |This section is a wholly new sub-clause of the original Declarative Environment Records clause.
329 |The concrete Environment Record method HasTopicBinding for 332 | declarative Environment Records returns whether the Environment Record 333 | is a topic-binding environment. The value is *true* only if its 334 | BindTopicValues method has been called.
335 |This section is a wholly new sub-clause of the original Declarative Environment Records clause.
350 |The method BindTopicValues for declarative Environment Records may 352 | only be called on an Environment Record when it is not yet a 353 | topic-binding environment, after which it does become a topic-binding 354 | environment.
355 |This section is a wholly new sub-clause of the 376 | original Object Environment Records clause.
377 |An Object Environment Record may never have a topic binding.
380 |This section is a wholly new sub-clause of the 395 | original Global Environment Records clause.
396 |A Global Environment Record may never have a topic binding.
399 |This section is a wholly new sub-clause of the original 412 | Environment Records clause.
413 | 414 |It presumptively uses `%` as the placeholder token for the 415 | topic reference. This choice of token is not a final decision; `%` 416 | could instead be `^` or some other token.
417 |The topic binding of a declarative Environment Record 420 | immutably binds the topic reference `%` to one value of any 421 | ECMAScript language type (called the topic value or simply 422 | the topic), within that declarative Environment Record, at 423 | the time of the Environment Record's instantiation. The topic of a 424 | declarative Environment Record conceptually serves as the value that its 425 | lexical context is "about".
426 | 427 |A topic-binding environment is a declarative Environment 428 | Record that establishes a topic binding. 429 | The topic environment of the running execution context is its 430 | LexicalEnvironment's innermost Environment Record that is also a 431 | topic-binding environment (or *null* if no such Environment Record 432 | exists), as defined by the abstract operator GetTopicEnvironment.
433 | 434 |An Environment Record is a topic-binding environment 436 | only when it is a declarative Environment Record that 437 | was created by a |PipeBody|.
438 |This section augments the original 448 | Execution Contexts clause.
449 |EvaluateWithTopics creates a new topic-binding environment and 472 | evaluates the given _expression_ with that as its topic environment. 473 | The previous Lexical Environment is restored afterward.
474 |GetTopicEnvironment returns the running execution context's topic environment 492 | (i.e., its LexicalEnvironment's innermost topic-binding environment) 493 | or *null* if the running execution context has no topic binding.
494 |GetPrimaryTopicValue returns the topic value 510 | of the running execution context's topic environment. 511 | It may be called only when the running execution context's topic environment 512 | is not *null*.
513 |This section augments the original 527 | Punctuators clause.
528 | 529 |It presumptively uses `%` as the placeholder token for the 530 | topic reference. This choice of token is not a final decision; `%` 531 | could instead be `^` or some other token.
532 |This section augments the original 570 | Primary Expression clause.
571 | 572 |It presumptively uses `%` as the placeholder token for the 573 | topic reference. This choice of token is not a final decision; `%` 574 | could instead be `^` or some other token.
575 |This section is a wholly new sub-clause to be inserted between the original `this` 602 | Keyword clause and the original 604 | Identifier Reference clause.
605 | 606 |It presumptively uses `%` as the placeholder token for the 607 | topic reference. This choice of token is not a final decision; `%` 608 | could instead be `^` or some other token.
609 |The topic reference, which is the token `%`, is a 613 | nullary operator that evaluates to the current Environment Record's 614 | topic value. The topic reference acts as if it were a special variable, 615 | implicitly bound to the topic value, yet still lexically scoped. But 616 | `%` is not actually an |IdentifierName| and the topic reference is 617 | not a variable, and it cannot be bound by typical assignment. 618 | Instead, the topic reference is immutably bound to a value 619 | during the instantiation of any topic-binding environment by a |PipeBody|.
620 | 621 |The concept of the topic binding is further discussed in
An unbound topic reference is a topic reference that 629 | is not present within any topic-binding environment created by a |PipeBody|. 630 | All unbound topic references are invalid syntax. 631 | Several early error rules for |ScriptBody| and for 632 | |ModuleItemList|, as well as a step in CreateDynamicFunction, 633 | use ContainsOuterTopic to check for any unbound topic reference `%`. 634 | Any inner topic reference within a |PipeBody| is hidden from these rules, 635 | preventing them from triggering the rules during program 636 | compilation.
637 | 638 |This guarantees that every topic reference in a program must be 639 | present within a topic-binding environment 640 | created by a |PipeBody| within that program.
641 |A topic reference may be evaluated only when 647 | the running execution context's topic environment is not *null*. 648 | This is syntactically enforced by early error rules 649 | for |ScriptBody| and for |ModuleItemList|, 650 | as well as a step in CreateDynamicFunction. 651 | These rules use ContainsOuterTopic to check for any unbound topic reference.
652 |This section is a wholly new sub-clause to be inserted between the original 668 | Conditional Operator (`?` `:`) clause and the original 670 | Assignment Operators clause.
671 |This section is a wholly new sub-clause.
686 | 687 |It presumptively uses `%` as the placeholder token for the 688 | topic reference. This choice of token is not a final decision; `%` 689 | could instead be `^` or some other token.
690 |A |PipeBody| must use its topic at least once. 699 | `value |> foo + 1` is an early error, 700 | because ContainsOuterTopic of its |PipeBody| is *false*. 701 | This design is such because omission of any topic reference from a |PipeBody| 702 | is almost certainly an accidental programmer error.
703 |A |PipeBody| must not be an unparenthesized |AssignmentExpression|, such as |YieldExpression|, |ArrowFunction|, or |ConditionalExpression|—unless it is a |ShortCircuitExpression|.
721 |This is to prevent confusing expressions from being valid, such as:
722 |
723 | x |> yield % |> % + 1 // Syntax Error
724 | This expression would otherwise be equivalent to:
726 |
727 | x |> (yield % |> % + 1)
728 | Likewise, this expression:
730 |
731 | x |> y ? % : z |> % + 1 // Syntax Error
732 | …would otherwise be equivalent to:
734 |
735 | x |> (y ? % : z |> % + 1)
736 | These expressions are visually unclear and are therefore made invalid. The developer may make them valid with explicit parentheses:
738 |
739 | x |> (yield %) |> % + 1
740 | x |> (yield % |> % + 1)
741 | x |> (y ? % : z) |> % + 1
742 | x |> (y ? % : z |> % + 1)
743 | This section augments the original 770 | Scripts clause.
771 | 772 |It presumptively uses `%` as the placeholder token for the 773 | topic reference. This choice of token is not a final decision; `%` 774 | could instead be `^` or some other token.
775 |An early error rule uses ContainsOuterTopic 789 | to check for any unbound topic reference. 790 | Any inner topic reference within a |PipeBody| is hidden from this rule, 791 | preventing them from triggering the rule during program 792 | compilation.
793 | 794 |This guarantees that every topic reference in a |Script| must be 795 | present within a topic-binding environment created 796 | by a |PipeBody| within that |Script|.
797 |This section augments the original 811 | Module Semantics clause.
812 | 813 |It presumptively uses `%` as the placeholder token for the 814 | topic reference. This choice of token is not a final decision; `%` 815 | could instead be `^` or some other token.
816 |An early error rule uses ContainsOuterTopic 831 | to check for any unbound topic reference. 832 | Any inner topic reference within a |PipeBody| is hidden from this rule, 833 | preventing them from triggering the rule during program 834 | compilation.
835 | 836 |This guarantees that every topic reference in a |ModuleBody| must be 837 | present within a topic-binding environment created 838 | by a |PipeBody| within that |ModuleBody|.
839 |This section augments the original 863 | CreateDynamicFunction clause.
864 | 865 |It presumptively uses `%` as the placeholder token for the 866 | topic reference. This choice of token is not a final decision; `%` 867 | could instead be `^` or some other token.
868 |