# Is it possible to use structural\_parameters as an argument of \`System\` in ModelingToolkit?
**URL:** https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396
**Category:** Modelling & Simulations
**Tags:** modelingtoolkit
**Created:** [October 23, 2025, 5:27pm UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396 "2025-10-23T17:27:19Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@pmc4](https://discourse.julialang.org/u/pmc4)
#### Post date: [October 23, 2025, 5:27pm UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396/1 "2025-10-23T17:27:19Z")
Hello everyone. When building a model in ModelingToolkit, there is the option of doing it with the macro `@mtkmodel`. Inside the macro, we can define structural parameters for defining numerical variables. For example,
```julia
const me = 0.511
@mtkmodel MyModel begin
@structural_parameters begin
mass = me
end
@variables begin
y(t)
end
@equations begin
y ~ exp(- 1 / t) * mass
end
end
```
Is it possible to do the same with the `System` constructor? It doesn’t have a `structural_parameters` keyword argument. From reading the docs, I haven’t found anything else yet.
I’m interested in knowing this because for the equations I need to define, it’s simpler to do so with the `System` approach. However, such equations depend on some parameters that are not symbolic, just constants, and some fixed matrices.
---
### Author:  [@cstjean](https://discourse.julialang.org/u/cstjean)
#### Post date: [October 24, 2025, 2:34am UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396/3 "2025-10-24T02:34:37Z")
I think that structural parameters are not stored in `System`. Eg.
```julia
julia> @mtkmodel Foo begin
@structural_parameters begin
x
end
end;
julia> @named f = Foo(x=3)
Model f:
julia> f.x
ERROR: ArgumentError: System f: variable x does not exist
```
Have a look at eg.[ModelingToolkitStandardLibrary.jl/src/Hydraulic/IsothermalCompressible/components.jl at main · SciML/ModelingToolkitStandardLibrary.jl](https://github.com/SciML/ModelingToolkitStandardLibrary.jl/blob/main/src/Hydraulic/IsothermalCompressible/components.jl#L69-L82). The structural parameters like `add_inertia` are not stored.
---
### Author:  [@pmc4](https://discourse.julialang.org/u/pmc4)
#### Post date: [October 24, 2025, 7:36am UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396/4 "2025-10-24T07:36:29Z")
You are right, they are not stored. I’ve created two simple examples, one with `@mtkmodel`
```julia
@mtkmodel Foo1 begin
@structural_parameters begin
λ = 4.5
end
@variables begin
y(x)
end
@equations begin
y ~ - λ * x
end
end
@named foo1 = Foo1()
```
and one with the `System` constructor
```julia
function Foo2(; name)
λ = 4.5
vars = @variables begin
y(x)
end
eqs = [
y ~ - λ * x
]
return System(eqs, x; name = name)
end
foo2 = Foo2(name = :foo2)
```
In both cases I get the same equation when calling to `equations(foo1)` or `equations(foo2)`:
```julia
1-element Vector{Equation}:
y(x) ~ -4.5x
```
I guess what is happening is that `@structural_parameters` are stored directly in the equations via their numerical variables, and there is no need to store them since they won’t change, they are not symbolic. Does it make sense?
---
### Author:  [@cstjean](https://discourse.julialang.org/u/cstjean)
#### Post date: [October 24, 2025, 8:38am UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396/5 "2025-10-24T08:38:45Z")
Yeah. In general the point of structural parameters is generally to change the structure (the number of components, the number of variables, the presence of some equations). I don’t see why they couldn’t be stored, though. Would be useful and consistent.
---
### Author:  [@pmc4](https://discourse.julialang.org/u/pmc4)
#### Post date: [October 24, 2025, 9:25am UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396/6 "2025-10-24T09:25:17Z")
Thanks a lot!
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [October 24, 2025, 12:43pm UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396/7 "2025-10-24T12:43:02Z")
They could be stored, but the issue is that they are structural, and so changing them would change the number of equations and such… so `remake` wouldn’t necessarily be possible on them. So there would need to be other safe-guards and such which make them essentially different from other parameters. I think @cryptic.ax had ideas of saving them though? But it would need to be done right as the naive way would be really buggy.
---
### Author:  [@pmc4](https://discourse.julialang.org/u/pmc4)
#### Post date: [October 24, 2025, 4:15pm UTC](https://discourse.julialang.org/t/is-it-possible-to-use-structural-parameters-as-an-argument-of-system-in-modelingtoolkit/133396/8 "2025-10-24T16:15:21Z")
I see. Knowing that I can use any numerical variable inside the definition of the equations and then pass that to the `System` constructor is good enough for me. Those variables will not change once the model is built, so there is no problem with that.