Illegal discrete var assignement in if clause doesn't give error message

[Peter Aronsson]

The following model is illegal:

class IllegalDiscreteVarInIfClause
  Real x(start = 1.0, fixed = true);
  Boolean b;
equation
  der(x) = -time;
  if noEvent(x < 0.5) then
  b = true;
  else
  b = false;
  end if;
end IllegalDiscreteVarInIfClause;

Because of the following statement in the modelica language specification:
Inside an if-expression, if-clause or for-clause, that is controlled by a non-discrete time switching expression
and not in the body of a when-clause, it is not legal to have assignments to discrete variables, equations between
discrete-time expressions, or real elementary relations/functions that should generate events. [This restriction is
necessary in order to guarantee that there all equations for discrete variable are discrete-time expressions, and to
ensure that crossing functions do not become active between events.] (Version 3.3 page 32.

This does not produce an error message in OpenModelica 1.9.0 beta2

[Jan Brugård]

This corresponds to Wolfram MathCore ticket 3932.

[Martin Sjölund]

I was going to write something about this being the backend's job since you cannot know for sure if the variable is continuous or not in the frontend. But then I saw the noEvent().

Is noEvent the only way to make continous boolean expressions? If so it should be feasible to fix this.

[Jan Brugård]

It is possible to make in other ways, e.g.

class IllegalDiscreteVarInIfClause
  Real x(start = 1.0, fixed = true);
  Boolean b;
  Boolean b2=<some continous expression>
equation
  der(x) = -time;
  if b2 then
  b = true;
  else
  b = false;
  end if;
end IllegalDiscreteVarInIfClause;

Independent of this, this should still be a frontend issue as there are a lot of things that the front-end needs to check depending on the variability of expressions (discrete or continuous).

[Martin Sjölund]

No, that b2 is a discrete-time expression according to the specification:

3.8.3 Discrete-Time Expressions
Discrete-time expressions are:

• Parameter expressions
• Discrete-time variables, i.e., Integer, Boolean, String variables and enumeration variables, as well as Real variables assigned in when-clauses

I believe this means that b2 is sampled whenever an event occurs, but it still is discrete. Find some other way of creating continuous boolean variables other than noEvent :)

[Martin Sjölund]

Postponed until 1.9.1

[Martin Sjölund]

This ticket was not closed for 1.9.1, which has now been released. It was batch modified for milestone 1.9.2 (but maybe an empty milestone was more appropriate; feel free to change it).

[Martin Sjölund]

Milestone changed to 1.9.3 since 1.9.2 was released.

[Martin Sjölund]

Moved to new milestone 1.9.4

[Martin Sjölund]

Milestone pushed to 1.9.5

[Martin Sjölund]

Milestone renamed

[Martin Sjölund]

Ticket retargeted after milestone closed

[Martin Sjölund]

Milestone moved to 1.12.0 due to 1.11.0 already being released.

[Francesco Casella]

Replying to brugard:

It is possible to make in other ways, e.g.

class IllegalDiscreteVarInIfClause
  Real x(start = 1.0, fixed = true);
  Boolean b;
  Boolean b2=<some continous expression>
  ...

I may be wrong, but I can't see how a continuous expression could be assigned to a boolean variable, which is discrete by definition.