Asymmetry between --omit-empty and --omissible-lists

[sjakobi]

In #1475 @astynax discovered the following asymmetry between --omissible-lists on the dhall-to-{json,yaml} side and --omit-empty in {json,yaml}-to-dhall:

omit-empty also "nullifies" not only empty arrays but also empty objects. omissible-lists translates only null ~> [] but not null ~> {}. So omit-empty and omissible-lists are not complete opposites.

It would be nice to increase the symmetry, but I'm not sure how…

[astynax]

Can we extend omissible-lists to smth like omissible-collections? This extended form will make empty objects from null as it does for arrays for now.

[sjakobi]

Yeah, a generalization like that should work. 👍

Let's see if we can find a better name than --omissible-collections…

[sjakobi]

Thinking more about this, I find it hard to imagine a situation where you'd use yaml-to-dhall to generate a Dhall config containing {=}, so I'm not sure whether it's worth implementing the missing null -> {=} translation.

I also haven't come up with a better option name than --omissible-collections – so I guess it's good enough! --omit-empty should be kept as a synonym for compatibility though.