Stage 1 Draft / April 8, 2018

Pipeline operator

Introduction

This document specifies the pipeline operator |>. See the explainer for an introduction.

The main design decisions made in this specification are:

  1. The right argument of |> is treated directly as a function; even if it is a CallExpression, the result of evaluating that CallExpression is treated as the function, rather than slotting the left argument into the arguments list. That is x |> y(z) is similar in semantics to y(z)(x). bug
  2. The precedence of |> sits between the ternary operator and ||. It is left-associative. bug
  3. All calls to eval are indirect eval.
  4. The pipeline operator is eligible for proper tail calls, when appropriate.
  5. The left argument to |> is evaluated before the right one.
  6. If the right argument to |> has a receiver, it is used for the call, with rules similar to a method call. That is x |> y.z gets y as a receiver, just as if it were written y.z(x).

1Syntax

ConditionalExpression[In, Yield, Await]:LogicalORExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield, ?Await]?AssignmentExpression[+In, ?Yield, ?Await]:AssignmentExpression[?In, ?Yield, ?Await] PipelineExpression[?In, ?Yield, ?Await] PipelineExpression[?In, ?Yield, ?Await]?AssignmentExpression[+In, ?Yield, ?Await]:AssignmentExpression[?In, ?Yield, ?Await] PipelineExpression[In, Yield, Await]:LogicalORExpression[?In, ?Yield, ?Await] [~Await]PipelineExpression[?In, ?Yield, ?Await]|>LogicalORExpression[?In, ?Yield, ?Await] [+Await]PipelineExpression[?In, ?Yield, ?Await]|>[lookahead ∉ { await }]LogicalORExpression[?In, ?Yield, ?Await] [+Await]PipelineExpression[?In, ?Yield, ?Await]|>await

2Semantics

2.1Runtime Semantics: PipelineEvaluate

PipelineEvaluate for lhs and rhs performs the following steps:
  1. Let argRef be the result of evaluating lhs.
  2. Let arg be ? GetValue(argRef).
  3. Let funcRef be the result of evaluating rhs.
  4. Let func be ? GetValue(funcRef).
  5. If Type(funcRef) is Reference, then
    1. If IsPropertyReference(funcRef) is true, then
      1. Let thisValue be GetThisValue(funcRef).
    2. Else the base of funcRef is an Environment Record,
      1. Let refEnv be GetBase(funcRef).
      2. Let thisValue be refEnv.WithBaseObject().
  6. Else Type(funcRef) is not Reference,
    1. Let thisValue be undefined.
  7. If Type(func) is not Object, throw a TypeError exception.
  8. If IsCallable(func) is false, throw a TypeError exception.
  9. Let thisCall be this outer PipelineExpression.
  10. Let tailPosition be IsInTailPosition(thisCall).
  11. If tailPosition is true, perform PrepareForTailCall().
  12. Let result be Call(func, thisValue, « arg »).
  13. Assert: If tailPosition is true, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
  14. Assert: If result is not an abrupt completion, then Type(result) is an ECMAScript language type.
  15. Return result.
Note
All calls to eval from a pipeline operator are indirect eval calls.
 Editor's Note
This definition is somewhat duplicated from the definition of evaluating CallExpression; when merging with the main specification, a refactoring will be done to de-duplicate it.

2.2Runtime Semantics: Evaluation

PipelineExpression:PipelineExpression|>LogicalORExpression
  1. Let result be the result of PipelineEvaluate for PipelineExpression and LogicalORExpression.
  2. Return result.
PipelineExpression:PipelineExpression|>await
  1. Let argRef be the result of evaluating PipelineExpression.
  2. Let result be ? GetValue(argRef).
  3. Return ? AsyncFunctionAwait(result).
PipelineExpression:LogicalORExpression
  1. Return the result of evaluating LogicalORExpression.

2.3Static Semantics: HasCallInTailPosition

PipelineExpression:PipelineExpression
  1. If this outer PipelineExpression is call, return true.
  2. Return false.

ACopyright & Software License

Copyright Notice

© 2018 Daniel Ehrenberg

Software License

All Software contained in this document ("Software") is protected by copyright and is being made available under the "BSD License", included below. This Software may be subject to third party rights (rights from parties other than Ecma International), including patent rights, and no licenses under such third party rights are granted under this license even if the third party concerned is a member of Ecma International. SEE THE ECMA CODE OF CONDUCT IN PATENT MATTERS AVAILABLE AT https://ecma-international.org/memento/codeofconduct.htm FOR INFORMATION REGARDING THE LICENSING OF PATENT CLAIMS THAT ARE REQUIRED TO IMPLEMENT ECMA INTERNATIONAL STANDARDS.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  3. Neither the name of the authors nor Ecma International may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE ECMA INTERNATIONAL "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL ECMA INTERNATIONAL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.