In Haskell, is there an abstraction for the <?>-operator?

I think it's a good idea to leave things at (<?>) = flip fromMaybe.

If you'd like to generalize though, Foldable seems to be the simplest class with a notion of emptiness:

(<?>) :: Foldable t => t a -> a -> a
ta <?> a = foldr const a ta 

This returns a if ta is empty or else the first element of ta. Examples:

Just 0 <?> 10 == 0
Nothing <?> 0 == 0
[] <?> 10 == 10

Actually, I don't like <?> operator name for what you are looking for. If you search for Maybe a -> a -> a type on Stackage or Hayoo you can find ?: operator from errors package.

And this operator is so called elvis-operator. It is used in Groovy in that form. And Kotlin also had it. This operator helps to deal with nulls in imperative language. But if you imagine that Maybe a is some kind of nullable type then ?: operator makes sense for you as well. You can observe fact that there is some history behind ?: operator.

Also, <?> is already used in some packages like megaparsec, attoparsec, optparse-generic and others. And your project may use one of those with high probability. So you may experience some conflicts using your version of elvis-operator.

The only two abstraction of "emptyness" I can think on top of my head are:

First, MonadError: Maybe could have instance MonadError () Maybe. See however https://github.com/ekmett/mtl/issues/27

Second, lens _Empty, which is by default comparison (Eq) with mempty (of Monoid). However Monoid and Alternative disagree for Maybe.

Yet I don't recall any operator directly working on either one above.