ERROR: Indexing with parameters is deprecated. Use `getp(A, eB)` for parameter indexing.

[orebas]

I had some working software, which stopped working with the above error.
When I google the error, nothing shows up.

General complaint: I find ModelingToolkit is very hard to use because the API changes frequently and sometimes in ways that break code silently. The interface for parameters has changed multiple times in the past two years, which causes a maintenance burden on every downstream package.

Specific complaint: I can't easily find documentation about the above change. It breaks my code and I don't know what version of ModelingToolkit to revert back to. Also, it seems like a pointless change which removes a feature for little to no benefit.

In the spirit of constructive feedback, I would recommend

1. that deprecating reasonable seeming features should be perceived as fairly onerous, and that a link to a serious justification (which could be a github issue or discourse discussion) is included in the error.
2. Before a deprecation becomes an error, it should give an obnoxious warning for a 6-18 months, with a link to a discussion.

[hersle]

It's at the top of the FAQ. See also the discussion in e.g. #3267.

I support your feedback, but I also think it is important to recognize that the API needs iterative reshaping to become as good as possible. MTK solves a very general problem, and converging towards the optimal interface is not a trivial process.

[ChrisRackauckas]

MWE? I can't do anything with no example of the code.

[orebas]

I am very confused. Here are two lines of code

newstates[s] = ode_solution[Symbol(state_param_map[s])][end]           #version (1)
newstates[s] = getp(ode_solution, Symbol(state_param_map[s]))[end]      #version (2) 

Just before posting this issue, version(1) had been working for me for... probably at least 6 months, maybe more.
I replaced it with (2), after getting that error referenced in the title.
Today I updated MTK, and now (2) gave an error; I reverted back to (1) and it is running.
It is pretty hard for me to reconstruct what versions of MTK I have progressed through, but right now I have
[961ee093] ModelingToolkit v9.64.0
[2efcf032] SymbolicIndexingInterface v0.3.37
[d1185830] SymbolicUtils v3.15.0
[0c5d862f] Symbolics v6.29.0

This issue is, I confess, a bit of a frustration vent. Many Julia packages seem to operate under a self-fulfilling assumption that there are few users who are not "in the loop". So the main source for a lot of info, including about breaking changes, is literally slack channels, which don't get recorded anywhere and are not searchable.

In the spirit of constructive criticism, I would maybe consider that "breaking" changes come in different flavours. Adding functionality or fixing clear bugs, while technically breaking sometimes, are less likely to actually break other packages. Removing functionality or changing the meaning or ordering of existing function signatures feels more like a major version change, (or rarely something to do intentionally because you know something doesn't work right).

For reference, just before (1) and (2) I had these lines.

state_param_map = (Dict(x => replace(string(x), "(t)" => "")
					for x in ModelingToolkit.unknowns(model.system)))

[ChrisRackauckas]

I still can't do anything with this. Can you give me an example?

So the main source for a lot of info, including about breaking changes, is literally slack channels, which don't get recorded anywhere and are not searchable.

There doesn't seem to be a breaking change here. From my understanding there is a bug report?

[AayushSabharwal]

Just before posting this issue, version(1) had been working for me for... probably at least 6 months, maybe more.

That's weird. Directly indexing solutions with parameters has been deprecated for a long time.

Today I updated MTK, and now (2) gave an error; I reverted back to (1) and it is running.

Did it give the same error?

For reference, just before (1) and (2) I had these lines.

state_param_map = (Dict(x => replace(string(x), "(t)" => "")
					for x in ModelingToolkit.unknowns(model.system)))

Why? It looks like you're trying to index a solution with a string representation of the variables. In that case, MTK.parse_variable is the function to use. It is documented in the FAQ here. Once you recover the symbolic variable from that, it can be used to index the solution.

As suggested previously, it's difficult to make progress on resolving your issue with the current description of your problem. It would be helpful if you can give a copy-pasteable code snippet which reproduces the error you're referring to.