Premise: In D language, only one main(){} function can exist in a program.
Having two main() functions throws an error.

Let's say I want to use some functionality of another program, but it has a main(){}
function. How can I import and use functions without importing the main(){} function?

On Tuesday, 8 March 2022 at 17:47:47 UTC, BoQsc wrote:

You'll have to modify the other program to exclude the main function from compilation.

For example, you could use a version condition:

module otherprogram;

version (Otherprogram_NoMain)
{
    // no main function
}
else
{
    void main(string[] args)
    {
        // ...
    }
}

// other functions...

Then, when you're compiling the program that uses it, you can pass -version=Otherprogram_NoMain on the command line:

$ dmd -version=Otherprogram_NoMain myprogram.d otherprogram.d

This will include the main function from myprogram.d, but exclude the one from otherprogram.d.

I think D Language needs and lacks conditional compilation condition and attribute of "exclude". The exclude keyword or code block in the exclude, would be made sure to not be imported by any means. Now it seems all to be only workarounds.

On Tuesday, 8 March 2022 at 20:12:40 UTC, BoQsc wrote:

What D just needs is a way to specify the entry point, in which it just defaults to the first main function found, but could be any function given.

On Tuesday, 8 March 2022 at 22:28:27 UTC, bauss wrote:

Yeah that would be as great. Without this, the project modules feel non-interactive and always depending on other modules. Dependant modules do not feel modular, if you can't bring them to other project as a standalone module.

On Tuesday, 8 March 2022 at 22:28:27 UTC, bauss wrote:

I just place main in a separate skeletal entrypoint.d file that contains only it (and an import to the "main" main.d/app.d/whatever), so dub can safely exclude it in unittest builds without skipping any other tests I might have in the main file.

mainSourceFile "source/myproject/entrypoint.d"

module myproject.entrypoint;

int main(string[] args)
{
    import myproject.main : run;
    return run(args);
}

I think D Language needs and lacks conditional compilation condition

On Tuesday, 8 March 2022 at 22:28:27 UTC, bauss wrote:

Which is similar to what Java does.

When i was first learning Java in a company i would make main() and have it run all the unittests of that particular module, then have a different file that actually combined all the tools together to run the program. Though when making the jar I'd specify which one actually was needed. But this was... 10 years ago.