Reporting intermediate solutions

[IgnaceBleukx]

For several use-cases where you solve hard optimization problems, it might be useful to get the intermediate solutions back from the solver.
I know Gurobi and OR-Tools support this feature through callbacks, but it requires a user to write its own wrapper for each solver he wants to use.

I think it would be nice if we can handle this in a more unified way.
If you agree, let's use this issue on how exactly this feature should be implemented.

My take:
Something similar to the display parameter in solveAll would be nice I think.
Here, we can either pass a (list of) expressions whose value is printed after each solution found, or a "real" callback function that gets called on every solution. A user can use the callback himself to track time until this solution or similar stats.

[ThomSerg]

Would definitely be nice in my opinion. For the XCSP3 competition this would have been useful, since as you said now I had to create a custom wrappers for each solver so that each prints intermediate results in the same consistent way.

Maybe we also want some support code so that it is much easier for the user when they do want a custom callback but without knowing all the solver-specific details on how to do this. So abstract some of the solver-specific details for writing a callback (optionally) away (for example all the code that is needed for Gurobi to check that the callback comes from the MIP solver). Maybe this is what you already meant.

[IgnaceBleukx]

Yes, that's indeed more or less what I envisioned : ) I'll make a branch and port some of the things you did for the competition onto that one.

[hbierlee]

In minizinc-python, you can actually iterate over the solutions, see https://python.minizinc.dev/en/latest/advanced_usage.html#concurrent-solutions, which might also be a nice design instead of callbacks:

async for result in instance.solutions(all_solutions=True):
   ...