Single-parameter linear constraints

[AdrianSosic]

We might consider forbidding the creation of single-parameter linear constraints since the same logic can be activated by placing appropriate parameter bounds. Going via constraints could have negative effects for the optimizer?

[AVHopp]

I'm against this since interpoint constraints make the most sense with a single parameter: "For my batch, I can in total only use 100ml of this substance". If we keep the design of interpoint constraints as they currently are (that is, by having a field is_interpoint), then we should at least allow single-point interpoint constraints.
If we want to re-design interpoint, we can still do that (currently waiting for #448 as I still need to test the interplay between interpoint and cardinality constraints)

[Scienfitz]

Sounds reasonable for normal use cases of parameters/cosntraints being fully controlled by suers.

But how about other situations like ones that can happen eg in cardinality context: In situations like described in #448 it might happen that a single parameter constraint is created by the parameter dropping and not the user. The solution to adjust bounds would require a subtantially more complicated logic than just using the single param constraint I guess?

[AdrianSosic]

@AVHopp: yes, interpoint excluded from what I said 👍🏼

@Scienfitz: Agree. Whatever we do, the outcome should be "we are certain that the optimizer is not constrained in unnecessary ways". And there are three ways to achieve this – which is the right one remains to be figured out:

• Perhaps no adjustment is needed at all, if things are already properly passed down to the numerical routines even in the current state.
• If not, we might still keep the single-parameter constraint interface for convenience but make sure that we pass it on in the right way to the consuming internal code
• Or, the approach you mentioned, where single-parameter constraints really need to be turned into adjusted parameters on the fly.

Let's see what we figure out.