Advise on dependencies

[hannesm]

Dear everyone, I have the following puzzle: mirage unikernels exercise dune code that is barely used, which allows us to get fresh unique issues that are fixed eventually.

The latest occurence was the interplay with lwt 6. For the time being, mirage-runtime is restricted to lwt < 6. But with dune 3.22 this sjould be leveraged.

Now, what is the right constraint to put on mirage-runtime? Can we express "lwt, but < 6 if dune < 3.22 is used"? Or do we have to select one (i.e. specizly lwt < 6 OR specify dune > 3.22), and cut the package universe? (since there are various packages conflicting with dune 3.22, maybe that's not the smartest idea?)

If you like to read on about the issue, please follow the following link ocaml/dune#13299 and don't be afraid to click on further references there.

Any input is welcome, I guess that @mseri, @kit-ty-kate or @jmid may have some ideas 💡, but I'm happy to hear any input. /cc @mirage/core

[kit-ty-kate]

yes this is reasonably easy to do:

"dune" {your normal dune constraints}
"lwt"  {your normal lwt constraints}
"lwt" {< "6"} | ("lwt" & "dune" {>= "3.22"})

(this is off the top of my head while i'm on the go, i might have made a mistake in the formula but it should be something like that, there are plenty of similar examples around in opam-repository if you need some)

[mseri]

I had come to suggest the same as @kit-ty-kate. I think it is the less restrictive and most convenient solution

[kit-ty-kate]

actually you can even simplify this line further if you don't need to add an extra constraint to lwt in the "dune >= 3.22" case

-"lwt" {< "6"} | ("lwt" & "dune" {>= "3.22"})
+"lwt" {< "6"} | "dune" {>= "3.22"}

also note that some opam solvers (e.g. 0install) at present gives a slight priority¹ to the first element(s) of a disjunction, so you can even optimize for lwt6 by flipping the two clauses:

-"lwt" {< "6"} | "dune" {>= "3.22"}
+"dune" {>= "3.22"} | "lwt" {< "6"}

Footnotes

1. it's a bit more complicated than that but i'm simplifying for the sake of telling you about it. Curious people can read more about in ocaml/opam#6826 ↩

[hannesm]

Thanks a lot. I'll do some local testing that the dune version really fixes the issue and propose a PR soon.