alt Hacker NewsIn the early drafts, we played with a number of approaches for the structure. Things like "commit-meta", etc. In the end, we broke it down into `#<section_level><section_type>`, just to simplify the parsing requirements. Every meta block is a meta block, and knowing what section level you're supposed to be in and comparing to what section level you get become a matter of "count the dots".
The header formats are meant to be very simple key/value pairs that are known by the parser, and not free-form bits of metadata. That's what the "meta" blocks are for. The parsing rules for the header are intentionally very simple.
JSON was chosen after a lot of discussion between us and outside parties and after experimentation with other grammars. The header for a meta block can specify a format used to serialize the data, in case down the road something supplants JSON in a meaningful way. We didn't want to box ourselves in, but we also don't want to just let any format sit in there (as that brings us back to the format compatibility headaches we face today).
For the other notes:
1. Compatibility is a key factor here, so we'd want to go with base-level JSON. I'd prefer being able to have trailing commas in lists, but not enough to make life hard for someone implementing this without access to a JSON5 parser.
2. If your goal is to simply feed to GNU patch (or similar), you can still split it. This extra data is in the Unified Diff "garbage" areas, so they'll be ignored anyway (so long as they don't conflict, and we take care to ensure that in our recommendations on encoding).
If your goal is to split into two DiffX files, it does become more complicated in that you'd need to re-add the leading headers.
That said, not all diff formats used in the wild can be split and still retain all metadata. Mercurial diffs, for example, have a header that must be present at the top to indicate parent commit information. You can remove that and still feed to GNU patch, but Mercurial (or tools supporting the format) will no longer have the information on the parent commit.
3. Revisions depend heavily on the SCM. Some SCMs use a commit identifier. Some use per-file identifiers. Some use a combination of the two. Some use those plus additional information that either gets injected into the diff or needs to be known out-of-bounds. There's a wide variety of requirements here across the SCM landscape.
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> 1. Compatibility is a key factor here, so we'd want to go with base-level JSON. I'd prefer being able to have trailing commas in lists, but not enough to make life hard for someone implementing this without access to a JSON5 parser.
This is what I was referring to. This is not json:
> #..meta: format=json, length=270
> The header formats are meant to be very simple key/value pairs that are known by the parser, and not free-form bits of metadata. That's what the "meta" blocks are for. The parsing rules for the header are intentionally very simple.
Exactly my point. That level of flexibility for a .patch format to support another language embedded in it is overwhelming. Keep in mind that you are proposing a textual format, not a binary format. So people will use 3rd party text parsing tools to play with it. And having 2 distinct languages in there makes that annoying and a pain.
What was your reasoning for discarding the existing header format used by git?
> Compatibility is a key factor here, so we'd want to go with base-level JSON. I'd prefer being able to have trailing commas in lists, but not enough to make life hard for someone implementing this without access to a JSON5 parser.
Everyone has access to a JSON5 parser. Everyone has to suffer for the sake of a few people who don't to pay the trifling tax of pip installing something --- when they're using an external library for a novel file format _anyway_?
> The header formats are meant to be very simple key/value pairs that are known by the parser, and not free-form bits of metadata. That's what the "meta" blocks are for.
One more thing you should prepare for whenever you have "free-form bits of metadata". They somehow turn into: "some user was storing 100MB blobs in there, and that broke our other thing".