A piece of advice I read somewhere early in my career was "a boolean should almost never be an …

If you use keyword arguments then something like that doesn't look too bad: serialize(someObject, prettyPrint:true) NB I have no idea whether Lua has keyword arguments but if your language does then that would seem to address your particular issue?

It's a failing of many type systems of older languages (except Pascal). The best way in many languages for flags is using unsigned integers that are botwise-ORed together. In pseudocode: Object someObject; foo (someObject, Object.Flag1 | Object.Flag2 | Object.Flag3); Whatever language you are using, it probably has some namespaced way to define flags as `(1 << 0)` and `(1 << 1)` etc.

But this isn't really a boolean problem - even in your example there is another mistery argument: nil And you can get the same problem with any argument type. What do the arguments in copy(obectA, objectB, "") mean? In general, you're going to need some kind of way to communicate the purpose - named parameters, IDE autocomplete, whatever - and once you have that then booleans are not worse than any other type.

> I now believe very strongly that you should virtually never have a boolean as an argument to a function. There are exceptions, but not many. Really? That sounds unjustified outside of some specific context. As a general rule I just can't see it. I don't see whats fundamentally wrong with it. Whats the alternative? Multiple static functions with different names corresponding to the flags and code duplication, plus switch statements to select the right function? Or maybe you're making some other point?

Named arguments are a solution to precisely this issue. With optional arguments with default value, you get to do precisely what was being done in your Lua code but with self documenting code. I personally believe very strongly that people shouldn’t use programming languages lacking basic functionalities.

Something I really love in Elixir is that functions can be named identically and are considered different with different arity. https://elixirschool.com/en/lessons/basics/functions#functio...

I die a little inside every time I write: JSON.stringify(val, null, 2); (So yes, but it goes beyond booleans. All optional parameters should be named parameters.)

I'm surprised nobody has suggested this yet. Just use a different name for the function. In your example, the new function should be prettyPrint(). No booleans required. No extra structures required.