Proposal - Preserve

[TotalTechGeek]

Background

Apologies for the deluge of simultaneous proposals.

As it looks like: https://github.com/orgs/json-logic/discussions/28 has succeeded, I'm going to propose one more operator for "Core".

After this, I think I'm going to personally try to avoid adding additional operators to JSON Logic Core, although there is a case to be made for a few simple string operators like startsWIth and endsWith... Though those could become a community extension rather than a Core operator.

Anyway, getting to the point:

Some folks have found it useful to be able to embed data in their logic.

json-logic-js had a quirk where if an object had >1 keys, it would be treated as embedded data, though this wasn't standard across most implementations of JSON Logic. However, this prevented you from returning objects with a single key, unless it was embedded in context.

In json-logic-engine, this is handled with a preserve operator, which intentionally preserves the argument and performs no transformation work on it.

Proposal

I'd like to suggest we add a preserve operator, which is a lazy operator which simply returns its argument without any evaluation or processing.

Put another way, preserve is a lazy operator that simply executes the identity function on its argument.

This operator makes it simple to embed data (specifically objects or arrays) in the AST, without it being executed.

Tests

preserve

[
    {
        "description": "Preserve on a string",
        "rule": { "preserve": "a" },
        "result": "a",
        "data": null
    },
    {
        "description": "Preserve on a number",
        "rule": { "preserve": 1 },
        "result": 1,
        "data": null
    },
    {
        "description": "Preserve on a zero",
        "rule": { "preserve": 0 },
        "result": 0,
        "data": null
    },
    {
        "description": "Preserve on a negative number",
        "rule": { "preserve": -1 },
        "result": -1,
        "data": null
    },
    {
        "description": "Preserve on true",
        "rule": { "preserve": true },
        "result": true,
        "data": null
    },
    {
        "description": "Preserve on false",
        "rule": { "preserve": false },
        "result": false,
        "data": null
    },
    {
        "description": "Preserve on an array",
        "rule": { "preserve": [1, 2] },
        "result": [1, 2],
        "data": null
    },
    {
        "description": "Preserve on a null",
        "rule": { "preserve": null },
        "result": null,
        "data": null
    },
    {
        "description": "Preserve on a single key object",
        "rule": { "preserve": { "a": 1 } },
        "result": { "a": 1 },
        "data": null
    },
    {
        "description": "Preserve on a multi key object",
        "rule": { "preserve": { "a": 1, "b": 2 } },
        "result": { "a": 1, "b": 2 },
        "data": null
    },
    {
        "description": "Preserve on an array of objects",
        "rule": { "preserve": [{ "a": 1 }, { "b": 2 }, { "a": 1, "b": 2 }] },
        "result": [{ "a": 1 }, { "b": 2 }, { "a": 1, "b": 2 }],
        "data": null
    },
    {
        "description": "Preserve on something that looks like an operator",
        "rule": { "preserve": { "+": [1, 2, 3] } },
        "result": { "+": [1, 2, 3] },
        "data": null
    },
    {
        "description": "Preserve on an empty array",
        "rule": { "preserve": [] },
        "result": [],
        "data": null
    },
    {
        "description": "Preserve on an empty object",
        "rule": { "preserve": {} },
        "result": {},
        "data": null
    }
]

[TotalTechGeek]

Self +1

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