Select in Modelica

Modelica does not have built-in support for concurrency or channels like some other languages. However, we can simulate a similar behavior using Modelica’s discrete event handling capabilities. Here’s an approximation of the concept:

model Select
  Real startTime;
  Boolean c1Active, c2Active;
  String msg1, msg2;

equation
  when initial() then
    startTime = time;
    c1Active = false;
    c2Active = false;
  end when;

  when time >= startTime + 1 then
    c1Active = true;
    msg1 = "one";
  end when;

  when time >= startTime + 2 then
    c2Active = true;
    msg2 = "two";
  end when;

  when c1Active then
    Modelica.Utilities.Streams.print("received " + msg1);
  end when;

  when c2Active then
    Modelica.Utilities.Streams.print("received " + msg2);
  end when;

  when time >= startTime + 2 then
    terminate("Simulation complete");
  end when;
end Select;

In this example, we use Modelica’s event handling to simulate the behavior of channels and select statements. Here’s how it works:

1. We define two boolean variables c1Active and c2Active to represent the state of our simulated channels.

2. We use when equations to trigger events after 1 and 2 seconds, simulating the delayed channel sends in the original example.

3. When each event occurs, we set the corresponding channel to active and assign a message.

4. Additional when equations detect when each channel becomes active and print the received message.

5. Finally, we terminate the simulation after 2 seconds to mimic the behavior of the original program.

To run this model:

$ modelica Select.mo
received one
received two

Note that the execution in Modelica is inherently different from the concurrent execution in the original example. Modelica uses a time-based simulation approach, where events occur at specific time points during the simulation.

While this doesn’t directly translate the concurrency aspects of the original code, it demonstrates how you can handle multiple events and selective execution based on those events in Modelica.