This might seem obviously “yes” at first, but consider a method like foo.debugRepr() which outputs the string FOO and has documentation which says it is meant only to be used for logging / debugging. Then you make a new release of your library and want to update the debug representation to be **FOO**.
Based on the semantics of debugRepr() I would argue that this is NOT a breaking change even though it is returning a different value, because it should only affect logging. However, if someone relies on this and uses it the wrong way, it will break their code.
What do you think? Is this a breaking change or not?
This one is a bit tricky, because you have to think about logging as an output or a side-effect. And as an industry, we’ve been learning that we should limit the amount of side-effects that our code generates.
If logging is getting ingested by downstream systems like CloudWatch, or other structured logging systems, it is potentially going to be used to detect service issues and track overall service health. These are logs that are serving a functional purpose that is not purely a side-effect, or for debugging forensics.
If this is the case, then you should have a unit test asserting that a log entry is emitted when a method is called. If writing that test is a low or non-priority, then even if it’s a “breaking change,” then that’s a sign that it’s not actually going to break anyone.
I’m sure there’s some monadic view of how to package up the “side-effect” logging as part of a function’s output, but it’s probably annoying to implement in most languages.