Proposal: Add tagged union to JSON Schema

[Ari Okkonen]

Tagged union is a way to define a data type that can have different but rigid structure for different purposes. It is useful e.g. in command lists to devices, and event reports from devices. Different commands for different actions require different kind but rigidly defined parameters, and different kinds of events produce different kind of data. Many programming languages support tagged unions (with different names e.g. discriminated unions). The Wikipedia article Tagged union describes the subject more thoroughly.

This is an example of a possible definition for variants:

{
  "type": "object",

  "tag": { 

    "myTag": {

      "type": "string", 

      "enum": ["move", "msg"] 

    }, 

  }, 

  "properties":{ 

    "id": { "type": "integer" } 

  }, 

  "variants":[ 

    { 

      "tagValue": "move", 

      "properties": {
        "x": { "type": "number" }, 

        "y": { "type": "number" } 

      } 

    }, 

    { 

      "tagValue": "msg", 

      "properties": { 

        "data": { "type": "string" } 

      } 

    } 

  ], 

  "additionalProperties": false 

}

This would pass the following records:

  { "myTag": "move", "id": "2780", "x": 758.32, "y": 334.28 } 

  { "myTag": "msg", "id": "2780", "data": "2A7F4A42B9022" }

but not:

  { "myTag": "move", "id": "2780", "data": "2A7F4A42B9022" } 

Ari Okkonen

[Jason Desrosiers]

This google group isn't used any more. Head over to our github (https://github.com/json-schema-org/json-schema-spec) or slack (json-schema.slack.com) if you want to pursue this. But, the short answer is that this this has been proposed and rejected multiple times. The main reason is that the pattern is already achievable with `if`/`then`.

[Ari Okkonen]

Thank you. Anyway, I'm glad you answered about the status of the group. Maybe a message with a title "GROUP RETIRED" and content directing to the new forum would help others.

BR

Ari Okkonen

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[Ben Hutton (@Relequestual)]

Hey Ari,

It isn't that the group is retired... just very rarely used. =]