When are params and error data optional?

The JSON-RPC spec says that the params field of a request or notification is optional; and similarly for the data field on a response error.

However, in many cases omitting params is nonsensical: the user can't ask for a hover without telling us where! And conversely, in the few cases where params aren't necessary, they must be omitted (because there isn't even a param type!).

So the approach we have been taking in HLS is:

• If the metamodel specifies a parameter type for a method, then params must be present
• If the metamodel does not specify a parameter type, then params must be absent

Is this right? If so, could we say as much in the spec?

There is a similar question for error data: when it is specified in the metamodel that a method has an error data type, does that mean that any errors for that method must include error data, or is it still optional?

No implementation file or test is identified. Start by reviewing the JSON-RPC and LSP specification sections covering request params and error data; done means the specification explicitly resolves when each field is required, optional, or forbidden.