Add support for multi-linebreak-preservation

[fkirc]

This is a solution for #207.
I already tested it successfully on the following feature-branch of attranslate: fkirc/attranslate#64
This API-proposal isn't really clean, but I do not dare to make any breaking changes as an external contributor.

[eemeli]

I don't think I like this. On a conceptual level, the AST does not promise to retain the exact shape of the source string. Once it's been passed through once, it'll stay pretty close but even then the docs do include at least this note:

Due to implementation details, the library's comment handling is not completely stable. In particular, when creating, writing, and then reading a YAML file, comments may sometimes be associated with a different node.

So in a sense the library is usable as a YAML prettifier by parsing & stringifying a source string. And as a part of that, it currently cleans up extraneous white space between nodes by collapsing it into a single empty line. This PR muddies the situation by changing the logic to only dropping any trailing white space from those lines, while retaining the complete drop of any empty lines after a comment.

I think I'd like to see much wider support for this change before considering it further. Also, there's no realistic way of making it not a breaking change, and having both spaceBefore and spacesBefore is just not going to happen.

[fkirc]

I don't think I like this. On a conceptual level, the AST does not promise to retain the exact shape of the source string. Once it's been passed through once, it'll stay pretty close but even then the docs do include at least this note:

Due to implementation details, the library's comment handling is not completely stable. In particular, when creating, writing, and then reading a YAML file, comments may sometimes be associated with a different node.
So in a sense the library is usable as a YAML prettifier by parsing & stringifying a source string. And as a part of that, it currently cleans up extraneous white space between nodes by collapsing it into a single empty line. This PR muddies the situation by changing the logic to only dropping any trailing white space from those lines, while retaining the complete drop of any empty lines after a comment.

I understand that this PR is not the way things are currently working, therefore I think that we need to step back from this specific PR and instead debate on the general goals of this library.
Preserving the visual presentation of a YAML as much as possible is a valid use case, whereas "prettifying" is another valid use case.
Some users want to have a "prettifier" that aggressively reformats YAML to match some "standard form".
This is not the thing that attranslate wants, attranslate wants the former use case instead (preserve as much structure as possible).
So the crucial question is:

Do you want to support the "preservation use case", or do you only care about the "prettier use case"?

I think I'd like to see much wider support for this change before considering it further. Also, there's no realistic way of making it not a breaking change, and having both spaceBefore and spacesBefore is just not going to happen.

To make it not a breaking change, we would probably need an additional config-option.
I would add such config-option if there is a chance of getting something merged (not necessarily this API-proposal).

[fkirc]

Perhaps I should rephrase my question:

Is there anything I could do to improve support for the "preservation use case", without threatening the already existing "prettier use case"?

My point is not to merge this particular PR, but rather to find a direction that leads to an improved format-preservation.

[fkirc]

I am closing this PR because it is not a nice fit for the current API.
Nevertheless, I am still aiming to get this feature done in another form.