Defer in Modelica

The concept of defer doesn’t exist in Modelica, but we can achieve similar functionality using the when terminal() clause, which executes when a simulation terminates. This is not exactly the same as defer, but it’s the closest equivalent for cleanup operations.

model DeferExample
  import Modelica.Utilities.Files;
  import Modelica.Utilities.Streams;

  function createFile
    input String fileName;
    output Integer fileId;
  algorithm
    Modelica.Utilities.Streams.print("creating");
    fileId := Modelica.Utilities.Streams.open(fileName, "w");
  end createFile;

  function writeFile
    input Integer fileId;
  algorithm
    Modelica.Utilities.Streams.print("writing");
    Modelica.Utilities.Streams.writeFile(fileId, "data\n");
  end writeFile;

  function closeFile
    input Integer fileId;
  algorithm
    Modelica.Utilities.Streams.print("closing");
    Modelica.Utilities.Streams.close(fileId);
  end closeFile;

protected
  Integer fileId;

equation
  fileId = createFile("defer.txt");
  writeFile(fileId);

algorithm
  when terminal() then
    closeFile(fileId);
  end when;

end DeferExample;

In this Modelica example, we’ve created a model that simulates the behavior of the original code. Here’s how it works:

1. We define three functions: createFile, writeFile, and closeFile, which correspond to the functions in the original code.

2. In the equation section, we call createFile and writeFile. This is equivalent to the immediate execution in the original main function.

3. The closeFile function is called in a when terminal() clause. This is similar to the defer functionality in the original code, as it ensures that the file is closed when the simulation terminates.

To run this model:

$ modelica DeferExample.mo
creating
writing
closing

Note that error handling in Modelica is different from Go. In this example, we’ve simplified error handling. In a more robust implementation, you would use Modelica’s exception handling mechanisms.

It’s important to note that while this Modelica code achieves a similar result, the execution model is quite different from the original Go code. Modelica is primarily used for modeling and simulation of physical systems, so the concept of deferred execution doesn’t translate directly.