proposal: cmp: add CompareBool

[dsnet]

We just did a migration switching over usages of slices.SortFunc from func(T, T) bool to func(T, T) int and noticed that many comparisons between booleans suddenly became more complex.

Given that cmp.Compare can't be used on bools because bools are not ordered, I propose the addition of:

// CompareBool returns
//
//	-1 if x is less than y,
//	 0 if x equals y,
//	+1 if x is greater than y,
//
// where false is ordered before true.
func CompareBool[T ~bool](x, y T) int {
	switch {
	case x == false && y == true:
		return -1
	case x == true && y == false:
		return +1
	default:
		return 0
	}
}

Alternatively, we could add a helper function that converts false to 0 and true to 1.

\cc @bradfitz @danderson

[Jorropo]

Alternatively, I guess that has been propsed but I can't find it.
Make bool a type alias of uint1 which would be a 1 bit twos complement number. The usual number conversion rules would apply.

[robpike]

How often does sorting boolean values arise? Surprised to hear this is a pain point.

[robpike]

As for your suggestion of a conversion from bool to int, something I have used myself, it's so easy to do that I can't see it needs to be in the standard library. No one's going to get it wrong and it's less text than this reply of mine. Far less.

[jimmyfrasche]

I've sorted on bool many times, always on some property like "new" or "featured" that means it should jump to the top of a list presented to a person, but it's never been on just that property: always sort on flag then title/date/etc. I'm not sure how much help this would be in those scenarios unless there was also some kind of cmp.And(cmp.Bool(x.Flag, y.Flag), cmp.Compare(x.Title, y.Title))

I would love bool↔int conversions, though. Not hard to write but neither was min/max and those get to be builtins. I may have even written those more than I have various min/max's

[randall77]

Part of the problem with a bool->int function is that there are several. In the compiler we have two, one that converts to int64 and one that converts to int32.

[dsnet]

How often does sorting boolean values arise? Surprised to hear this is a pain point.

In our migration, ~20% of conversions required expanding the logic for boolean comparisons.

[ericchiang]

As another observation. I recently implemented a comparison for protobuf messages, and also wrote this by hand

[renthraysk]

This I think is one area where the compiler could do better*, the return early style in simple functions like this causes more branching than necessary, as it fails to realize it could be using conditional sets or moves.

The compiler can write this without branches.

func CompareBool[T ~bool](x, y T) int {
	r := 0
	if x != y {
		r = 1
		if y {
			r = -1
		}
	}
	return r
}

• Whether it's worth improving, I have no idea.

[benhoyt]

How often does sorting boolean values arise? Surprised to hear this is a pain point.

In our migration, ~20% of conversions required expanding the logic for boolean comparisons.

@dsnet Would you be able to give a couple of concrete examples of this?

[dsnet]

@benhoyt, our examples are very similar to what @jimmyfrasche mentioned. It's wanting to sort on boolean properties. Our situation is networking specific(e.g., whether DNS is set, whether an IP address is v4 or v6, whether a node is rejected, whether a key is revoked, etc.).

[robpike]

Maybe comparison operators should work on bools. They did in C. false < true.