The name focus comes from, so maybe more correct name would ifocused (c.f. itraversed).

-- (Actually, AffineTraversable, and Traversable1 would be more correct superclasses, if also existed).
class Traversable t => Singleton t where
    focus :: Functor f => (a -> f b) -> t a -> f (t b)

A name I invented just today, so I'm not (yet) attached to it.

https://oleg.fi/gists/posts/2018-05-12-singleton-container.html uses peekaboo :)

Is there a word for going to an amusement park, with only one decent ride, get on it, and then go home?

ifocus is a very generic name 🤔 I take it lens doesn't have this?

Maybe ipaired?

Or isecond? Then we can also add ifirst.

ifst and isnd would make sense. (first is slightly too overloaded). Also they are about as short as _1 and _2.

Sounds good 👍

@adamgundry are you fine with this?

@adamgundry no worries. As long as you find time to discuss breaking changes, there is no problem delaying things like this. :)