If I'm understanding you correctly, then fp-ts has the `constant` function to enable what you're describing in #1:

    fold(constant('my static fallback'), f)(x)

What sucks about this, as much as it's my preferred pattern, is that without native pattern matching in the language it's unclear which callbacks are for which type constructors. It's not so bad for something like Either, but if you're using something like DatumEither with ~five constructors it's a bit of a mess.