[GitHub] [arrow-datafusion] kou commented on pull request #2622: Export minimum C API and examples for C, Ruby and Python

[GitBox <gi...@apache.org>]

kou commented on PR #2622:
URL: https://github.com/apache/arrow-datafusion/pull/2622#issuecomment-1140777959

   Regarding repository, I'm OK with it. Could you create https://github.com/datafusion-contrib/datafusion-c or something? Or should I create my repository for this?
   
   > I'm also curious why we would want to go from Python -> C -> Rust rather than just Python -> Rust directly as we do in https://github.com/datafusion-contrib/datafusion-python
   
   Sorry for confusing you. I didn't want to suggest that we should use this for Python bindings. I just wanted to show that we can use FFI library for bindings as an use case of this C API. I should have used Julia or something rather than Python.
   
   > * It would be nice to put a minimal readme in `datafusion/c` (saying, for example, that the directory contains the C API, see `examples/README.md` for more details). Or maybe we could move `datafusion/c/examples/README.md` to `datafusion/c/README.md` to make it more discoverable
   > 
   > * I think it would be a good idea to track planned follow on items as individual tasks
   
   They make sense. I'll do them in new repository for this.
   
   > I am not familiar how to interface Rust `async` functions with C -- I would assume it looks like callbacks somehow, but I can imagine how it gets very tricky very quickly
   
   I will not use callbacks like the following:
   
   ```c
   int
   main(void)
   {
     DFSessionContext *context = df_session_context_new();
     DFError *error = NULL;
   
     DFDataFrameFuture *sql_future =
       df_session_context_sql_async(context, "SELECT 1;", &error);
     if (error) {
       printf("failed to start SQL: %s\n", df_error_get_message(error));
       df_error_free(error);
       df_session_context_free(context);
       return EXIT_FAILURE;
     }
   
     DFDataFrame *data_frame = df_data_frame_future_await(sql_future, &error);
     if (error) {
       printf("failed to run SQL: %s\n", df_error_get_message(error));
       df_error_free(error);
       df_session_context_free(context);
       return EXIT_FAILURE;
     }
   
     DFFuture *show_future = df_data_frame_show_async(data_frame, &error);
     if (error) {
       printf("failed to start showing data frame: %s\n",
              df_error_get_message(error));
       df_error_free(error);
       df_data_frame_free(data_frame);
       return EXIT_FAILURE;
     }
   
     df_future_await(show_future, &error);
     if (error) {
       printf("failed to show data frame: %s\n",
              df_error_get_message(error));
       df_error_free(error);
       df_data_frame_free(data_frame);
       return EXIT_FAILURE;
     }
   
     df_data_frame_free(data_frame);
     df_session_context_free(context);
     return EXIT_SUCCESS;
   }
   ```
   
   But I may change my mind.
   
   
   Thanks for suggestions for my Rust code! This is my first Rust program. So suggestions are very welcome. :-)
   


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