The way I see it, effects would be implemented/assigned to the function where the error gets logged for instance. But as long as the error value remains local,

The way I see it, effects would be implemented/assigned to the function where the error gets logged for instance. But as long as the error value remains local, a function can still be pure all else being equal. This is not the case with exception looking code (aka panics) since it escapes normal control flow and I guess makes an error "punch" through stacks as they are unwinded. A bit like being thrown toward global state. So if we consider panics as side effectful operations, we would have to change go to explicitly declare panics as side effects with a known function signature. I guess one would want the list of side effects to be part of a function signature for full visibility. I wonder how that would influence backward compatibility.