> I think booleans (flags) are unavoidable at times. Consider a case where you want a flag to switch verbose logging on and off selectively.

In my experience, even these situations tend to grow arms and legs pretty quickly, so you end up with a 'verbose' boolean that accepts four values: True, False, "superverbose" and "off". Then you notice that in some places the function is being called with an array as the value for 'verbose', because you're in a language where arrays are implicitly cast to Boolean (I'm looking at you python).

There are too many cases where the optional parameter is not really a boolean, but merely a sum type which coincidentally only has two members at the moment, but is likely to grow to more members.