just thinking aloud, not a strong opinion by any means -- what do you think about running Validate here, and panicing if an error is returned? it's programmer error in that case, since it means they called Run without Validate, and a panic with a message seems better than a panic caused by a nil later?

In the "correct" case, where a developer calls Validate before Run, then it's never possible for Validate to fail, so there won't be a panic. The only time a panic should occur is when the caller forgets to call Validate before Run, and in that case getting a stack trace and error message could be helpful

the downside of course is that Validate is run twice, but those checks should be relatively inexpensive, so that seems fine to me

That's actually much nicer.

I don't like the idea of users getting a faceful of stacktrace though, so adding a recover() at the top-level.