Add the --preserveNull flag and omit null fields by default on dhall-to-yaml

[mujx]

Currently this is a breaking change because the --omitNull flag is removed, but it would seem a bit confusing to keep it and have two flags for the same purpose. Let me know if I should keep it.

$ dhall-to-yaml --preserveNull <<< '{ a = "dhall", b = None Natural, c = [ ] : List Text }'
a: dhall
b: null
c: []

$ dhall-to-yaml <<< '{ a = "dhall", b = None Natural, c = [ ] : List Text }'
a: dhall
c: []

$ dhall-to-yaml --omitEmpty <<< '{ a = "dhall", b = None Natural, c = [ ] : List Text }'
a: dhall

[sjakobi]

Many thanks for the PR! :)

Currently this is a breaking change because the --omitNull flag is removed

I'm confused. The code for parsing that option is still there.

IMHO it's OK to keep --omitNull for a bit, for backwards compatibility, but we should probably add a TODO as a reminder to eventually remove it.

Can you also update this example?

dhall-haskell/dhall-json/src/Dhall/JSON.hs

Lines 144 to 157 in 75e6cc5

	> $ cat ./example.dhall
	> let JSON = https://prelude.dhall-lang.org/JSON/package.dhall
	>
	> in JSON.object
	> [ { mapKey = "foo", mapValue = JSON.null }
	> , { mapKey =
	> "bar"
	> , mapValue =
	> JSON.array [ JSON.number 1.0, JSON.bool True ]
	> }
	> ]
	> $ dhall-to-json <<< './example.dhall'
	> {"foo":null,"bar":[1,true]}

[mujx]

The code is still there because it's still used by dhall-to-json which continues to have both --omitNull and --omitEmpty.

Unless I should also update dhall-to-json.

[sjakobi]

Ah, that makes sense! 👍

Sorry for the confusion!

[Gabriella439]

I think it's worth keeping the --omitNull flag (even as a no-op) for at least one release to allow for a smooth migration path

Also, even thought I only requested this for dhall-to-yaml this discussion makes me realize that we should probably do the same for dhall-to-json, too

[sjakobi]

LGTM, apart from one more wibble. 👍 :)

[sjakobi]

@Gabriel439 I think this is fine to merge. Could you give it a quick look?

[Gabriella439]

Yeah, this looks great to me! I just added the merge-me label

@sjakobi: For future reference, you can merge without my approval. If I see anything that still needs to be fixed I can always create a follow-up pull request