Wouldn't the use of named parameters solve the confusion aspect that's cited as the reason to not use boolean parameters? Not that the use of enums or distinct function wrappers is a bad approach (it's probably more elegant), but the hash approach does eliminate quite a bit of ambiguity.

With languages that include full pattern matching support and atom/symbol literals (like Erlang and friends, for example, and like most other functional languages IIRC), you could alternately match on a particular symbol instead of relying on a boolean flag.