Http Client in Modelica

Here’s an idiomatic Modelica example that demonstrates the concept of making an HTTP client request:

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

  function sendHTTPRequest
    input String url;
    output String response;
  external "C" response = sendHTTPRequest(url)
    annotation(Include="#include <curl/curl.h>",
               Library="curl");
  end sendHTTPRequest;

equation
  when initial() then
    // Send an HTTP GET request
    String response = sendHTTPRequest("https://example.com");
    
    // Print the response status (assuming it's the first line)
    Streams.print("Response status: " + Streams.readLineTokens(response, 1)[1]);
    
    // Print the first 5 lines of the response body
    for i in 1:5 loop
      Streams.print(Streams.readLineTokens(response, i + 1)[1]);
    end for;
  end when;
end HTTPClient;

This Modelica example demonstrates how to make an HTTP client request, similar to the concept shown in the Go example. Here’s an explanation of the code:

1. We define a model called HTTPClient.

2. We import necessary Modelica utilities for file and stream operations.

3. We declare an external C function sendHTTPRequest that takes a URL as input and returns the response as a string. This function is meant to use the libcurl library to perform the actual HTTP request.

4. In the equation section, we use a when initial() clause to execute the HTTP request when the simulation starts.

5. We call sendHTTPRequest with the URL “https://example.com” and store the response.

6. We print the response status, assuming it’s the first line of the response.

7. We then print the first 5 lines of the response body using a for loop.

Note that Modelica doesn’t have built-in HTTP client functionality like Go does. This example uses an external C function that would need to be implemented separately using a library like libcurl. The actual implementation of sendHTTPRequest is not provided here, as it would require C code and isn’t part of standard Modelica.

To use this model:

1. Save the code in a file named HTTPClient.mo.
2. Implement the external C function sendHTTPRequest using libcurl.
3. Compile the model using a Modelica compiler that supports external C functions, such as OpenModelica or Dymola.
4. Run the simulation.

This example demonstrates how Modelica can interface with external libraries to perform tasks that are not natively supported, such as making HTTP requests. It follows Modelica conventions and best practices, using equation-based modeling and external function calls.