[chucksmash]

I was replying to this:

> IMO this counts as a kind of out-of-band arrangement because it doesn't involve using one of the normal return values to signal error.

My point is that if you're returning an algebraic data type Result that encodes Ok(val) or NotOk(_), you are using one of the normal return values, because the function always returns a Result. The whole purpose of the construct seems to me to be bringing the error case handling into band.

[theoh]

The way I see it, each of the data types in a sum type is analogous to a channel. The type of the 'message' conveyed by a value is effectively "Either Control Data".

In-band signalling is messier than out-of-band signalling, so I see out-of-band as the goal. If you look at the various approaches to the semipredicate problem on that Wikipedia page, it looks like, historically, we started out with awkward in-band solutions and the evolution is towards the clarity offered by expanding the return type to send errors out-of-band.

I mentioned this previously on HN, and maybe that earlier comment will clarify what I'm talking about: https://news.ycombinator.com/item?id=18766478

pjc50's way of applying the terms to flow of control is a somewhat different thing.