Differentiate types for "Pull Request Simple" and "Pull Request" to support richer data in `Github_core.Pull.get`

[shonfeder]

Currently, both Github_core.Pull.get and Gitub_core.Pull.for_repo both return values of type pull.

My understanding is that, type pull represents what the GitHub API (in the response schema for the repos/OWNER/REPO/pulls endpoint) calls a "Pull Request Simple".

However, the repos/OWNER/REPO/pulls/PULL_NUMBER endpoint (for fetching a single pull request) returns what GitHub calls in the response schema a "Pull Request".

Critically for my purposes, the "Pull Request" object includes many properties that are omitted from the "Pull Request Simple" object, including properties that indicate whether a PR is mergable or whether it has auto_merge enabled.

If we wanted to support inspecting these extended attributes, I think we'd need to introduce a new ATD type that inherited from and extended type pull, and then adapt Github_core.Pull.get to deserialize into the corresponding record.

Perhaps this new type could be something like the following?

type pull_full = {
  mergeable: bool;
  (* etc... *)
  inherit pull
}

If maintaining backwards compatibility where not paramount, you might instead rename the current type pull as type pull_simple to more closely reflect the GitHub API.

On the other hand, to avoid breaking backwards compatibility, we might introduce a new function Github_core.Pull.get_full.

What do you all think of pursuing some such change?

[tmcgilchrist]

@shonfeder backwards compatibility is important, I would pursue the first option of introducing a new type and follow that implementation. Atd allows for inheriting properties from one type to another, which might be useful in this case.
eg https://github.com/mirage/ocaml-github/blob/master/lib_data/github.atd#L132-L135