Should schema.type be always defined?

[relu91]

TODO: introduce better the problem @JKRhb

Regarding the use cases, please find a brief first attempt (that definitely needs more work) for the four terms below:

oneOf

Relevant when a device described by a TD offers two different representations of the same resource (e.g., a property or an action output). A concrete example is the things property in the Thing Description Directory API, where – depending on the URI variable format – the directory can return TDs either as an array or a "collection", i.e. as part of a JSON object.

const

As a developer, I might want to specify that a Thing always returns a certain value when an action is performed. A simple example could be a value like "Success". Combined with oneOf, we could describe that an interaction (in this case an action) always either returns "Success" or "Operation failed" like so:

{
  "actions": {
    "exampleAction": {
      "output": {
        "oneOf": [
          {
            "const": "Success"
          },
          {
            "const": "Operation failed"
          }
        ]
      }
    }
  }
}

default

As a developer, I want to save bandwidth, especially in resource-constrained environments. Therefore, I want to be able to specify a default value that has to be assumed by a TD consumer if the response sent after an operation has been completed does not contain a value.

enum

As a developer, I want to constrain the values that are valid as an interaction output to a set of values. This could be used to describe the allowed states of a smart light ("on" or "off"), for example.

Originally posted by @JKRhb in #534 (comment)