Why is it append instead of replace when adding fields

[3tbraden]

I was trying to have some common fields in all logs and I used .With() and .WithOptions for that. The problem is, when I pass the logger to a child function, which logs the same field again, the value in the field is repeated.

i.e.

func fnA() {
  logger = logger.With(zap.String("s", "sth"))
  ctx = NewContext(ctx, logger)
  fnB(ctx)
}

func fnB(ctx) {
  logger = FromContext(ctx)
  logger.Info("s", "else")
}

the log from fnB above will become {"s": "sthelse"}. Is it possible to replace instead of append when adding the fields? Any reason why it should be append?

[abhinav]

Hey @3tbraden. Yeah, this is a fair question.
Part of the reason for the choice is this: it's difficult to get the performance we want out of Zap by doing this with replace.
Particularly, note that With encodes keys at the time logger.With is called.
Going back and changing what's already been encoded to fix a duplicate would require decoding and re-encoding that blob.

There's also been some prior discussion about this in #622.