Fix #604 Where applicable, use elif in Cabal files

[mpilgrem]

See:

• Where applicable, use elif in Cabal files #604

[sol]

This whole HasEmpty construction is not particularly beautiful.

I think what I would try is either

• try to transform nested conditionals into elifs at parsing time

or instead

• focus on some general solution to avoid staircases in package.yaml (as we started discussing in #604)

(or maybe even do both)

[sol]

I don't think that this is relevant user documentation. Nested if-statements and elif are semantically equivalent. As I understand it, this is a purely cosmetic change.

I say, either remove this completely, or add a separate section that describes how .cabal files are formatted. This section could then also include what I mentioned here: #606 (comment)

[sol]

What this is saying is that if a nested conditional can be transformed into an elif, then require Cabal 2.2.

What I think you would want to do is the other way around: If the rendered .cabal file has cabal-version: 2.2 (or later) then produce elifs.