Your first program

Everything from this level, in one small CLI.

Let’s build a complete little command-line tool using only what the basics covered: variables, math, functions, control flow, and the fallible model. It converts a Celsius temperature passed on the command line into Fahrenheit.

fn c_to_f(c: Float) -> Float {
    return c * 9.0 / 5.0 + 32.0;
}

fn main() {
    // arg(0) is the program name; arg(1) is the first real argument.
    let raw = std::env::arg_or(1, "20");

    let celsius = std::str::parse_float(raw) or {
        eprintln("not a number: ", raw);
        return;
    };

    let f = c_to_f(celsius);
    println(raw, "C = ", to_string(f), "F");
}

Run it:

hale run temp.hl 100

100C = 212F

With no argument it falls back to "20" and prints 20C = 68F — the tool self-demonstrates.

What each piece is doing

• std::env::arg_or(1, "20") reads command-line argument 1, or "20" if there isn’t one. (std::env::args_count() and std::env::arg(i) are the lower-level pair.)
• std::str::parse_float(raw) or { ... } addresses the fallible parse. Here the or arm prints to standard error and returns early — a fine motion when the success type is a value but you’d rather bail than substitute. (eprintln is println for stderr.)
• c_to_f is a plain free function — a calculation with no state, exactly what free functions are for.
• println(raw, "C = ", to_string(f), "F") concatenates its arguments. No format string.

This is a real program

You can hale build temp.hl and ship the resulting binary. It reads input, validates it, computes, and reports — and it’s honest about failure, because the parse had to be addressed. At this level Hale is a small, sharp scripting language.

You may have noticed there’s no locus here, no bus, none of the structural machinery from the introduction’s matchmaker. You don’t need it yet. A program that’s a handful of functions and a main is a perfectly good Hale program.

The next level is where structure starts to pay off — when your program grows state that lives over time, talks to the filesystem and the network, and wants to be organized into named parts. That’s where the locus earns its place.

Next: The locus, gently.