Value of JSON fields returned by the API

[lauraschauer]

Hi everyone, 

I'm trying to create a (somewhat) up-to-date OpenAPI3 specification (here) for the Canvas API, to use it with an OpenAPI3 Haskell code generator (here) to generate a Haskell client library for the API. 

My questions: 

Is it specified somewhere which fields in the JSON objects, that are returned from API calls, can be null and which cannot?

Is the list of fields marked with Optional: in the description of each object complete? 

What is the difference between nullable fields and optional fields? 

For example, in the documentation of a Profile object: 

// user, sample
"sortable_name": "user, sample",
"title": null,
"bio": null,
...
// Optional: This field is only returned in certain API calls, and will return
// the IANA time zone name of the user's preferred timezone.
"time_zone": "America/Denver",

Can I correctly infer from the documentation that 

• title and bio are always present in the response, and can take null as a value,
• on the other hand, time_zone is not always present in the response, and cannot take null as a value? 

Thank you in advance!