[bpicolo]

For 1/, you can return a struct value type without exporting it. If it satisfies the receiving interface they won’t have a problem.

That’s exactly the pattern I use for most Go development

[kbolino]

This affects discoverability, though. Your unexported type won't have public documentation. So you end up having to publish an interface anyway (even if you don't return it) or document in words what the method set looks like.