[whatwg/fetch] Redesign "extract header values" and "extract header list values" (#814)

These APIs had the wrong design and are also incorrectly used at times (#559). "Extract a MIME type" also needs a new design, likely on top of this as a new `Content-Type` parser: #529.

In particular, other than for `Set-Cookie` the ideal way to parse a header is to combine first, and then parse, which isn't really what the current operations are doing. How to combine headers is nearly settled: #813. Once that's done my tentative plan is to introduce a new "get" for header lists to solve #752 which would return null or the combined value for a given header name from a header list.

Another problem with the current setup is that the actual parsing is handwavy by referencing ABNF defined "somewhere", which in practice leads to it being poorly tested. We can continue to allow ABNF for headers where that is actually how they are implemented, but we need to be more strict about referencing the ABNF and writing the tests and not allowing as much indirection as is currently allowed for.

Unfortunately this will impact downstream, but I think it's the right thing to do long term and will lead to better considered header parsers (and encourage more reuse as well). I'll also start filing downstream issues now linking this issue to ensure we actually design something that works for everyone.

So I think what we want is basically "get" and then per header name we'll need to define its parser (which could just be an ANBF reference plus tests) or parsers (`Content-Type` differs between request and response, yay). Thoughts on the details here appreciated as well as on the overall plan of course.

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/814