In the general case, adding IO to any piece of code requires changing all callers. I would argue that it's a feature, not a bug: now `f x` is no longer a value,

In the general case, adding IO to any piece of code requires changing all callers. I would argue that it's a feature, not a bug: now `f x` is no longer a value, but an action: calling it twice can result in duplicated logs, for example. If you need that logging for debugging then you should use `Debug.trace` though.