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

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