OCaml 4.02-5.0 support

[dra27]

Alternate implementation to #831, retaining the current version matrix.

[dbuenzli]

I think we should move on the matrix. Why not make a last release for pre-4.08, the system is entirely usable for these (likely rare) people and then move to 4.08. See #868

[jonludlam]

I've been thinking about this a bit. I'd quite like to see the features we know we want for the ocaml website to land before we drop support for the older versions - so for example the 'uses' work that will be used to drive search. That way, once we've got those bits and pieces in we can just have a maintenance branch that supports 4.02-4.07 that will hopefully be easy to manage, and we won't have to backport that sort of relatively large feature.

Of course, if it actually does become more of a burden then we can revisit this, but so far keeping our lower bound at 4.02 has only been a case of a few minor shims here and there.

[dbuenzli]

That way, once we've got those bits and pieces in we can just have a maintenance branch that supports 4.02-4.07 that will hopefully be easy to manage, and we won't have to backport that sort of relatively large feature.

I guess that could be a reasonable reason. Though you could also question whether you really want to provide documentation that far in the past (and not, e.g. ensure that past documentation is available through archive.org) or not simply limit new features like search to post 4.08 or rely on web search engines prior to that.

Of course, if it actually does become more of a burden then we can revisit this, but so far keeping our lower bound at 4.02 has only been a case of a few minor shims here and there.

Let's not pretend those have not a cost (and if you are fine keeping minor shims here and there you should also be ok with adding minor functions to avoid introducing new dependencies ;-) But what made me actually open #868 was that the other day I was going to procrastinate by implementing #826 but got put-off by the fact that I couldn't use cmdliner's arbitrary command nesting.

so for example the 'uses' work that will be used to drive search.

Where can I get more information about that ? I've been wanting to add local search to odig for quite some time (which is the other reason why I want #826, to be able add more JavaScript to pages without disturbing upstream) so I'm quite interested on what the plans are there and if I can piggy back on them.

[dra27]

I wholeheartedly agree with dropping old compiler support, this (kind of) PR is only helpful to uncouple "supporting OCaml 5.x" from "changing the support lowerbound" as the latter tends to involve more discussion 🙂

[Julow]

Looks good!

Alternatively, we could implement the new API from the old API (this is what the formatting code does, Codefmt, using Marshal) but this is probably not a priority.

[jonludlam]

The ranked-search thing is here: #567