Many languages have a "null-coalescing" operator: a binary operator used to unwrap an optional/nullable value, or provide a "default" value if the LHS is null/none. It's usually spelled ?? (as in Javascript, Swift, C#, etc.).

I'm pondering the precedence of such an operator.

Why not just use no precedence? Parenthesis! S-expressions! Polish!

All interesting ideas! But this post will focus on a more "C-style" language perspective.

As for ??, it seems like there's a bit of variety. Let's start with a kind of basic operator precedence for a hypothetical C-style statically typed language with relatively few operators:

There are subtly differences here and there, but this is just for comparisons. Here's how (some) different languages handle the precedence.

• Below #5:

• C#

• PHP

• Dart

• Equal to #5

• Javascript (Kinda; ?? must be disambiguated from && and ||)

• Between #3 and #4:

• Swift

• Zig

• Kotlin

So, largely 2 camps: very low precedence, or moderately low. From a brief look, I can't find too much information on the "why" of all of this. One thing I did see come up a lot is this: ?? is analogous to ||, especially if they both short-circuit. And in a lot of programming languages with a looser type system, they're the same thing. Python's or comes to mind. Not relevant to a very strict type system, but at least it makes sense why you would put the precedence down that. Score 1 for the "below/equal 5" folk.

However, given the divide, it's certainly not a straightforward problem. I've been looking around, and have found a few posts where people discuss problems with various systems.

• https://forums.swift.org/t/nil-coalescing-operator-precedence/2954

• https://www.codeproject.com/Tips/721145/Beware-The-null-coalescing-operator-is-low-in-the

These seem to center around this construct: let x = a() ?? 0 + b() ?? 0. Operator precedence is largely cultural/subjective. But if I were a code reviewer, attempting to analyze a programmer's intent, it seems pretty clear to me that the programmer of this wanted x to equal the sum of a() and b(), with default values in case either were null. However, no one parses ?? as having a higher precedence than +.

This example might be a bit contrived. To us, the alternate parse of let x = a() ?? (0 + b()) ?? 0 because... why would you add to 0? And how often are you chaining null coalescing operators? (Well, it can happen if you're using optionals, but it's still rare). But, it's a fairly reasonable piece of code. Those links even have some real-world examples like this people have fallen for.

Looking at this from a types perspective, I came to this conclusion; In a strongly-typed language, operator precedence isn't useful if operators can't "flow" from high to low precedence due to types.

To illustrate, consider the expression x + y ?? z. We don't know what the types of x, y, and z are. However, if ?? has a lower precedence than +, this expression can't be valid in a strictly typed language, where the LHS of ?? must be of an optional/nullable type.

If you look back at our hypothetical start table, you can see how operator types "flow" through precedence. Arithmetic produces integers, which can be used as arguments to comparisons. Comparisons produce booleans, which can be used as arguments to logical operators.

This is why I'd propose that it makes sense for ?? to have a precedence, in our example, between 1 and 2. That way, more "complex" types can "decay" though the precedence chain. Optionals are unwrapped to integers, which are manipulated by arithmetic, decayed to booleans by comparison, and further manipulated by logic.

Discussion questions:

1. What are some reasons for choosing the precedence of ?? other than the ones discussed?

2. Have any other languages done something different with the precedence, and why?

3. Has anyone put the precedence of ?? above arithmetic?

Thanks!

I've recently had a couple of bugs using || instead ??, and I have a question about what kind of comparision these operators do.