when equations in algebraic loop?

[gkurzbach]

Is it allowed for when equations to be part of an algebraic loop?
Following model leads to an error in Dymola but not in OMEdit:

model algebraicLoop 
	Real x;
	Real y(start=0.5,fixed=true);
	equation
		x = time - y;
		when time>0.5 then
		    y = -x/2;
		end when;
end algebraicLoop;

I have not found any statement to this regard in the specification. Please clarify.

[HansOlsson]

It seems that it is allowed by the specification (there is even one example of it) , but that:

• Few libraries use it (that's why the lack of support hasn't been noticed).
• The specification misses the implications in some places, e.g., similarly to when initial() with impure function? #3499 there could here be a call of an impure function causing it to end up in a loop.

The when-equations are even designed to make it clear how such cases are handled. That's why you cannot write:

                x = time - y;
		when time>0.5 then
		    y + x/2 = 0;
		end when;

because it wouldn't make it clear which variables are updated as part of the when and which ones aren't.

[HansOlsson]

I believe the impure part can be handled in #3499 - The remaining part is thus just to state that there is nothing special with when-clauses and they can be part of systems of equations.

[henrikt-ma]

For what it's worth, System Modeler also builds and simulates this without any complaint.

[HansOlsson]

Dymola 2024x Refresh 1 also builds and simulates this without any complaint.

[HansOlsson]

I thus believe it was just a tool-issue, but it might be that the specification should be clearer.

[HansOlsson]

Language group: Agreement