Exposing a way to interpret and treat Solver diagnostics

[NathanReb]

It seems that the Solver.diagnostics is currently unexposed and the only API entry that allows dealing with this type is the eponymous function which simply returns it as a single string.

Is there something I could use to be able to programmatically inspect solver errors? If not would you be willing to add such a thing?

That would prove quite helpful for opam-monorepo, in particular to help with tarides/opam-monorepo#168 where we want to provide suggestions based on why the solving failed.

We could hack something by looking for a specific substring but I could settle for a cleaner solution!

[talex5]

You could probably just expose the underlying type provided by 0install, which organises things by package and has more structure:

https://docs.ocamllabs.io/packages/0install-solver/2.17/Zeroinstall_solver/Diagnostics/index.html#type-t

[NathanReb]

Sounds good, would you accept a PR adding this?

[NathanReb]

Hmm it seems that simply exposing the underlying type won't exactly do since the actual diagnostics type is defined as:

type diagnostics = Input.requirements

which if I made no mistake is `Zeroinstall_solver.S.CORE_MODEL.requirements.

The diagnostics function runs this through Solver.do_solve before pretty printing it. It would probably be better to expose a function that would return a Zeroinstall_solver.Diagnostics.t instead wouldn't it?

[talex5]

Ah, yes. The diagnostics are calculated lazily.

[NathanReb]

Sorry for the delay, I had a few other things to deal with first but I'm getting back to this as being able to inpertret the solver's result when there's no solution is proving more and more important for opam-monorepo!

I'll open a PR shortly and we can discuss the specifics there instead!