Warning
Info
It's not a problem since I can always just make a struct that gives me this functionality, but from a language design perspective, I am curious why HashSet doesn't implement PartialOrd. There is a very natural partial ordering on the collections of sets: the subset relation, which could be implemented as:
impl<T: Hash + Eq> PartialOrd for HashSet<T> {
fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
match self.len().cmp(&rhs.len()) {
Ordering::Equal if self.iter().all(|t| rhs.contains(t)) => Some(Ordering::Equal),
Ordering::Less if self.iter().all(|t| rhs.contains(t)) => Some(Ordering::Less),
Ordering::Greater if rhs.iter().all(|t| self.contains(t)) => Some(Ordering::Greater),
_ => None,
}
}
}
One could extend to the same idea to HashMap when the value type is PartialEq by replacing all(|t| rhs.contains(t)) with all(|(k, v)| rhs.get(k).is_some_and(|w| *w == *v)).
For even more flexibility, one could modify this for value types implementing PartialOrd by first comparing the underlying sets of keys and then performing a point-wise comparison of the values, but that might be getting too wild: in particular, it would conflict with the foregoing definition.
In any case, I'm just curious why the seemingly-natural definition of a partial order on sets is not part of std.
3 posts - 3 participants
🏷️ rust_feed