Hello, World

Create a file called hello.rue:

fn main() -> i32 {
    println("Hello, Rue!");
    0
}

Run it:

scripts/rue exec hello.rue

Hello, Rue!

Three things are happening here.

main is the entry point. Every Rue program has a function called main that takes no arguments and returns an i32.

The return value is the exit status. The last expression in a function body, written without a semicolon, is the function's value. Here that is 0, which becomes the process's exit status. Zero means success. Let's check:

scripts/rue exec hello.rue
echo $?

Hello, Rue!
0

println prints a line. It writes its argument followed by a newline to standard output. print does the same without the newline:

fn main() -> i32 {
    print("Hello, ");
    print("Rue!");
    println("");
    0
}

Hello, Rue!

Exit codes are values

Because main returns an ordinary integer, a program can report a result through its exit status. This program returns 3:

fn main() -> i32 {
    println("about to exit with 3");
    3
}

about to exit with 3

scripts/rue exec three.rue; echo $?

You will see the message and then 3. Several of the repository's examples work this way. It is handy in scripts, but note that a shell treats any nonzero status as failure, so the programs in this tutorial return 0 unless the exit status is the point.

Comments

Comments start with // and run to the end of the line:

fn main() -> i32 {
    // This is a comment.
    println("comments are ignored"); // so is this
    0
}

comments are ignored

Compiling by hand

scripts/rue exec is convenient, but it throws the executable away. To keep it, use the compiler directly (with RUE and RUE_STD_PATH set up as in the previous chapter):

"$RUE" hello.rue -o hello
./hello

The compiler takes one source file and an output path. Larger programs are made of several files, but you still name only one: the root. Chapter 12 explains how the rest are found.

A debugging aid

println prints text. When you just want to see a number while you are developing, the @dbg intrinsic prints any integer or boolean and a newline:

fn main() -> i32 {
    @dbg(42);
    @dbg(true);
    0
}

42
true

The @ prefix marks a compiler intrinsic: an operation the compiler provides directly rather than a function defined in some library. You will meet a few more. Use println for anything a user should see, and @dbg when you are poking at a value.

Next: values, types, and how to turn a number into something println can print.