Comment by AnimalMuppet

> Usually the flag is there to introduce a branching condition (effectively breaking "a function should do one thing")...

But if you don't let the function branch, then the parent function is going to have to decide which of two different functions to call. Which is going to require the parent function to branch. Sooner or later, someone has to branch. Put the branch where it makes the most sense, that is, where the logical "one-ness" of the function is preserved even with the branch.

> I find the same can be achieved w/ polymorphism and/or pattern-matching, the benefit being now your behaviour is part of the data model (the first argument) which is easier to reason about, document, and extend to new cases (don't need to keep passing flags down the call chain).

You just moved the branch. Polymorphism means that you moved the branch to the point of construction of the object. (And that's a perfectly fine way to do it, in some cases. It's a horrible way to try to deal with all branches, though.) Pattern-matching means that you moved the branch to when you created the data. (Again, that can be a perfectly fine way to do it, in some cases.)