Using Tuples within ModelingToolkit.jl?

General Usage
1

Trying to utlized MTK on a larger simulation and having trouble with functions that return tuples that are a function of state. For example:

function foo(x)
    return 1.0,2.0
end
@register_symbolic foo(x)

function plant(; name)
    @variables x(t)=1 u(t)=0 y(t)=0
    D = Differential(t)

    k1,k2 = foo(x); 

    eqs = [D(x) ~ -x*k1 + u;
              y ~ x*k2]
    ODESystem(eqs, t; name = name)
end

@named p = plant()

and receiving the error:

ERROR: BoundsError: attempt to access Num at index [2]

I realize this is a trivial example but demonstrates the point. In my actual implementation, the functions that rely on states are complex and have external dependencies (external c-library). I’ve tried writing simple wrappers but at a loss. How do we handle such cases?

2

I’m not sure it can handle tuple outputs in registered functions. That’s worth an issue. It would need to treat it internally as an array variable.