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jep-019-pipe-expressions-evaluation.md

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Evaluation of Pipe Expressions
JEP 19
Author Maxime Labelle
Created 29-October-2022
SemVer MINOR
Status accepted

Abstract

This JEP introduces changes that outline the exact intended behaviour of evaluating a pipe-expression. It also clarifies an ambiguity when evaluating a sub-expression where the left-hand-side evaluates to null.

Motivation

Sub Expression Evaluation

The specification for sub-expression outlines how it should be evaluated using pseudocode:

left-evaluation = search(left-expression, original-json-document)
result = search(right-expression, left-evaluation)

This is slightly ambiguous, as many compliance tests expect the result to be null when the left-hand-side evaluates to  null. So, the real pseudocode shoud in fact be:

left-evaluation = search(left-expression, original-json-document)
if left-evaluation is `null` then result = `null`
else result = search(right-expression, left-evaluation)

Pipe Expression Evaluation

In contrast, however, it seems intuitive for pipe-expression to evaluate as was originally outlined by the first pseudocode fragment referred to a above.

left-evaluation = search(left-expression, original-json-document)
result = search(right-expression, left-evaluation)

Which means that the evaluation should still happens if the left-hand-side is null.

Specification

Sub Expressions

The paragraph on sub expressions in the specification will be updated as follows, with changes outlined in bold:

A subexpression is evaluted as follows:

  • Evaluate the expression on the left with the original JSON document.
  • If the result of the left expression evaluation is null, return null.
  • Otherwise, eEvaluate the expression on the right with the result of the left expression evaluation.

In pseudocode:

left-evaluation = search(left-expression, original-json-document)
If result is `null` the result = `null`
else result = search(right-expression, left-evaluation)

Pipe Expressions

Likewise the paragraph on pipe expressions in the specification will be updated as follows:

A pipe expression combines two expressions, separated by the | character. It is similar to a sub-expression with twothe following important distinctions:

  1. Any expression can be used on the right hand side. A sub-expression restricts the type of expression that can be used on the right hand side.
  2. A pipe-expression stops projections on the left hand side for from propagating to the right hand side. If the left expression creates a projection, it does not apply to the right hand side.
  3. A pipe-expression evaluates the right expression unconditionally. A sub-expression shortcuts evaluation if the result of the left expression evaluation is null

In pseudocode:

left-evaluation = search(left-expression, original-json-document)
result = search(right-expression, left-evaluation)

Example

Expression Result
`null` . [@] null
`null` | [@] [ null ]

Compliance

The pipe.json compliance test file will be updated with the example from this JEP.