Just want to note that Rust has this; https://doc.rust-lang.org/std/cmp/struct.Reverse.html .

The 'sort' package already has a Reverse function for sort.Interface, so a Reverse function for the 'cmp' package would fit well in analogy. https://pkg.go.dev/sort#Reverse

I don't think T needs to be Ordered. This function can reverse any comparator on two values of any type.
https://go.dev/play/p/1TNHMQ7Cnoe

@itchyny: You're right. I'll update the proposal.

Just pondering here, should the implementation be fn(y,x) or -fn(x,y)? Maybe if fn is a strict weak ordering it doesn't matter?

@randall77: It's a fair thing to ponder. I believe they're equivalent.

Let fn be a function of type func(x, y T) int and suppose that an asymmetric relation $<$ on $T$ can be defined as follows. (Strict weak orders are asymmetric.)

• If fn(x, y) > 0, then $y<x$.
• If fn(x, y) < 0, then $x<y$.
• If fn(x, y) == 0, then neither $x<y$ nor $y<x$.

The sign of fn(y, x) is the opposite of the sign of fn(x, y).

• If fn(y, x) > 0, then $x<y$, so fn(x, y) cannot be positive (as it'd break the asymmetry) or zero (as $x<y$).
• If fn(y, x) < 0, then $y<x$, so fn(x, y) cannot be negative (as it'd break the asymmetry) or zero (as $y<x$).
• If fn(y, x) == 0, then neither $x<y$ nor $y<x$, so fn(x, y) cannot be positive or negative.

I quite like the implementation that swaps the arguments as it gets quite directly to the duality, but I'm not totally wedded to it.