main

If the entry file declares fun main, it runs after the top level. If it takes a parameter, that is the program’s arguments, an array of strings. It returns nil to exit with 0, or an exit code from 0 to 255.

// greet.rn
fun main(args) {
    if args.len() == 0 {
        io.eprint("usage: greet <name>...");
        return 2;
    }

    for name in args {
        print("hi " + name);
    }
}

$ rune run greet.rn ann bob
hi ann
hi bob
$ rune run greet.rn
usage: greet <name>...
$ echo $?
2

Arguments are always strings; convert them yourself:

// repeat.rn
fun main([word, times = "1"]) {
    print(word.repeat(int(times)));
}

$ rune run repeat.rn ab 3
ababab

The top level runs first, so main sees everything declared there. Without main the top level is the whole program.

let greeting = "hello";

fun main() {
    print(greeting); // hello
}

main takes at most one parameter. Returning an integer out of range is a ValueError, and anything else but nil or an integer a TypeError:

fun main() { return 256; } // ValueError: main must return nil or an exit code from 0 to 255, got 256.

fun main() { return "ok"; } // TypeError: main must return nil or an exit code from 0 to 255, got "ok".

To exit from deeper in the program, use os.exit(code).