Functions in Zig can be called both in runtime and in compile-time. You can force some expression to be called during comptime using a keyword, and sometimes the comptime is implied (like when you define top-level const)

If a function is called in comptime, it can also return types. So for example:

    // this is a function which accepts a type
    // if you accept type, you also have to restrict the arg to be comptime
    // if the arg is comptime it still does not mean that the function cannot be called in runtime,
    // but in this case, it returns type, so it is comptime-only function
    // there are also cases where this is not true, like std.mem.eql(u8, a, b) which accepts type,
    // but can be called in runtime because it does not return type
    fn Wrapper(comptime T: type) type {
        return struct { value: T };
    }

    const F32Wrapper = Wrapper(f32);

    // @TypeOf(x.value) == f32
    var x: F32Wrapper = .{ value: 1.0 }