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truthiness.md

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Background

In the draft, a filter expression is a predicate written within "[?" and "]". It evaluates to true or false (or invalid.) The nodes for which the predicate truth value is true are selected, the ones for which the predicate truth value is false are discarded.

To evaluate a filter expression, it is necessary to define rules to derive the truth value for an expression.

According to the rules in the draft, given the document

[{"foo": true},{"foo":false}]

the filter expressions appearing in the queries

$[?(@.foo)]
$[?(@.bar)]

are existence tests, and these queries produce

[{"foo": true},{"foo": false}]
[]

respectively.

The motivation for deriving the truth value for @.foo and @.bar as existence tests originates with Goessner's 2007 article, in particular, from this line

XPath JSONPath Result
//book[isbn] $..book[?(@.isbn)] filter all books with isbn number

Thereafter, implementations split.

  • Implementations that used JavaScript (or another dynamic language) for evaluating filter expressions evaluated @.foo and @.bar to truth values according to the rules of that language. These included Goessner's JavaScript implementation. While rules for defining truth value differed between languages, they generally agreed that @.foo evaluated to the value part of a name-value pair matching on 'foo', and that a value of true evaluated to true and a value of false evaluated to false.

  • 2011 Jayway JSONPath implemented it's own expression language and followed Goessner's original suggestion, as did NewtonSoft Json.Net.

As noted, the draft decided to follow Goessner's original suggestion, and be consistent with Jayway JSONPath and NewtonSoft Json.Net.

This comment proposes that that decision be revisited for the following reasons.

It's inconsistent with other JSON query languages.

All of these examples use the JSON document

[{"foo": true},{"foo":false}]

Draft

Query: $[?(@.foo)]

Result: [{"foo": true},{"foo": false}]

Query: $[?(@.bar)]

Result: []

Query: $[?(true)]

Result: Invalid

Query: [false]

Result: Invalid

XPath 3.1

Query: ?*[?foo]

Result: {"foo": true}

Query: ?*[?bar]

Result: No match

Query: ?*[true]

Result: {"foo": true}{"foo": false}

Query: ?*[false]

Result: No match

JMESPath

Query: [?foo]

Result: [{"foo": true}]

Query: [?bar]

Result: No match

Query: [?`true`]

Result: [{"foo": true},{"foo": false}]

Query: [?`false`]

Result: No match

JSONata

Query: *[foo]

Result: {"foo": true}

Query: *[bar]

Result: no match

Query: *[true]

Result: [{"foo": true},{"foo":false}]

Query: *[false]

Result: no match

the predicate expression is coerced to an xs:boolean value, called the predicate truth value

"Those items for which the predicate truth value is true are retained, and those for which the predicate truth value is false are discarded."

XML

Document:

Stamboul Train The Comedians

Query: //book[xs:boolean(@in-stock)]

Results:

Stamboul Train

Stamboul Train The Comedians

Query: //book[@isbn]

Results:

Stamboul Train

8838939810<title>Stamboul Train</title> <title>The Comedians<.title>

//book[isbn/text()]

8838939810<title>Stamboul Train</title>