Cannot create problems with tuple vector variables

[TorkelE]

using ModelingToolkit, OrdinaryDiffEq
using ModelingToolkit: t_nounits as t, D_nounits as D

@variables (X(t))[1:2]
@parameters p[1:2]
eqs = [
    D(X[1]) ~ p[1] - X[1],
    D(X[2]) ~ p[2] - X[2]
]
@named model = ODESystem(eqs, t)
model = complete(model)

# Problem inputs
u0_vec = [X => [1.0, 2.0]]
u0_dict = Dict([X => [1.0, 2.0]])
u0_tup = (X => [1.0, 2.0])
p_vec = [p => [4.0, 5.0]]
p_dict = Dict([p => [4.0, 5.0]])
p_tup = (p => [4.0, 5.0])
tspan = (0.0, 10.0)

# Problem creation:
ODEProblem(model, u0_vec, tspan, p_vec)
ODEProblem(model, u0_vec, tspan, p_dict)
ODEProblem(model, u0_vec, tspan, p_tup)
ODEProblem(model, u0_dict, tspan, p_vec)
ODEProblem(model, u0_dict, tspan, p_dict)
ODEProblem(model, u0_dict, tspan, p_tup)
ODEProblem(model, u0_tup, tspan, p_vec) # Error.
ODEProblem(model, u0_tup, tspan, p_dict) # Error.
ODEProblem(model, u0_tup, tspan, p_tup) # Error.

Probably holds for other problem types as well

[ChrisRackauckas]

I'm not sure this is supported? Dictionaries and things that transform to dictionaries are supported. Tuples of pairs I guess could work, we'd need to add it to to_dict. What's the use case here?

[TorkelE]

I think Tuples have always been supported, right? For parameters I think they are even required for when you have different types in your values. Right now I don't think it is a thing for variables, but seems weird to specifically disallow it here (especially as it might even be useful for hybrid ODE/Jump systems)

[ChrisRackauckas]

No, the parameters always uses an MTKParameters so it always constructs a different type-stable object and has a representation independent of the user code there. That's a requirement in general in order to support things like non-tunables.

[isaacsas]

Tuples were supported in MTK as a way to preserve the type of parameters for years. Then a breaking change was made that upconverted everything to floats in a non-breaking MTK8 release (but tuples were still supported as mappings). Catalyst has used tuple mappings forever because we needed mixed parameter types long before MTK9. There doesn’t seem to be any indication that MTK9 dropped tuple mapping support in the NEWS.md?

[ChrisRackauckas]

It's a part of the change to MTKParameters. But there's no reason to not support it, we might as well just convert it to a vector since that's always faster, and it would be faster for the user to just give a vector but we might as well collect general collections.

[ChrisRackauckas]

Wait this is for u0 being a tuple? We've never supported that, the differential equation solver needs vector operations in order to work.