Lifting binary operations, e.g. T+T → Option<T> + Option<T>

language design
27

The lift defined by @fweimer is for semigroups. None of your counterexamples are semigroups. But in that same respect, you have a point; I wouldn’t really call it a “lift”, either. It’s much too restrictive compared to your typical higher order abstraction. Applicative and monadic lifts like your Lift are so powerful because they support arbitrary functions without restriction.

Drawing a page from Haskell, I might sketch @fweimer’s idea into something more like this:

/// Trait for an associative operation
trait Semigroup: Sized {
    fn then(self, other: Self) -> Self;
}

// Things that could impl Semigroup
struct Max<T>(T);
struct Min<T>(T);

fn semigroup_opt<G: Semigroup>(a: Option<G>, b: Option<G>) -> Option<G>
{ ... }

(modulo weeks of futzing around to give it nicer ergonomics around borrowed data.)

And the generalization to more entries would merely reduce an IntoIterator<Item=Option<G>> into an Option<G>. (though it could also simply be written as a flattening operation followed by some IntoIterator<Item=G> -> Option<G>)