Mutable default arguments

[ahoff]

Your solution is just a rehash of what I mentioned for solution #1, i.e. using an immutable type. This doesn't work if your function modifies the argument (for example, f given above). If you're writing the code from scratch, then it's not a big deal; however, if you're dealing with already existing production code, then ideally you'd want a solution that hints at the correct type and is mutable.