Skip to content
FelsFels

Your first program

fels new sets up a project with a build file, a small program and a test:

Terminal window
fels new hello
cd hello
fels build run
fels test

This is the src/main.fels it writes for you:

#import std.io;


/// The sum of the numbers from 1 to `n`.
sum_to :: fn(n: i32) -> i32 {
    total := 0;
    i := 1;
    while i <= n {
        total += i;
        i += 1;
    }
    return total;
}


main :: fn() -> i32 {
    io::println("Hello from hello!");
    io::println("1 + 2 + ... + 10 = {sum_to(10)}");
    return 0;
}


#test
sum_to_adds_the_numbers :: fn(t: *Test) {
    t.eq(sum_to(0), 0);
    t.eq(sum_to(1), 1);
    t.eq(sum_to(10), 55);
}

A few things to notice:

  • name :: value declares a constant. A function is a constant whose value is a fn.
  • name := value declares a variable, and its type comes from the value.
  • Text in braces inside a string is printed in its place: {sum_to(10)}.
  • A #test function takes a *Test and checks things with it. fels test runs them all.
  • A /// comment documents the declaration below it. Your editor shows it on hover, and fels doc makes a reference from them.

For a single file without a project, fels run hello.fels builds and runs it in one step.

Where to go next

Section titled “Where to go next”
  • The overview walks through the whole language.
  • Memory and zones explains how memory works without a garbage collector.
  • Compile-time execution shows what #run can do.
  • The examples folder has a raytracer, a raycaster, a Win32 disk analyzer, a UEFI app and small kernels.