Hyrbid Modul and Program Behaviour for a D Source File

Question

A Python source file has the nice property of being able to act both as module or as a standalone program (tool) using the pattern

if __name__ == "__main__":

Is it somehow possible to get the same behaviour for a D module source file?

Answer 1

(Unix only)

You can use a shebang line that sets a version which enables a main function:

#!/path/to/rdmd --shebang -version=run
version(run) void main() {}

Make your file executable (chmod +x foo.d) and run it like a program (./foo.d).

Be sure to use a unique version identifier (unlike I did here). Maybe include the fully qualified module name in some form, or use a UUID.

Answer 2

It depends on what you're trying to do. A D program requires exactly one main function across all modules as the entry point, so there's not an implicit way as in Python. The D way is to create the executable as a separate module that contains a main and imports the other module.

But if you just want to do it for testing purposes, you should put executable code in unittest blocks (with no main) and then you can run the file using rdmd -main -unittest scratch.d, which adds a stub main for you.

If you really want to make a dual-purpose module (which is not really The D Way), you could put the main inside a unique version block:

module scratch; // file scratch.d
import std.stdio;

void foo(){ writeln("FOO"); }

version(scratchExe) {
    void main() {
        foo();
    }
}

Then compile the executable version with dmd scratch.d -version=scratchExe.