Built-ins

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Use these helpers for console input and output, basic calculations, time, and information about the running program. Console and system helpers need import system;; Math and primitive methods are available automatically.

Import: import system; for console, env, and process helpers. Math, Array, and primitive methods need no import.

import system;

fun main() {
    System.println("hello");
    System.println((-3.0).abs());
}

Console

Call	What it does
System.print(x)	write without a newline
System.println(x)	write, then newline
System.set_foreground / set_background / reset_color()	color until reset
System.clear()	clear the terminal
System.read_line()	one line of input; parse it with int.parse(line), double.parse(line), or bool.parse(line)
System.read_key()	one character
System.exit(code)	stop the process
System.panic(message)	print and halt — see Panics
System.stdin / stdout / stderr	byte streams implementing Reader / Writer (import system.io)

Platform and env

Call	What it does
System.platform / System.os_family	which host you are on; compare System.platform == Platform.Browser to detect a page
System.args	command-line arguments
System.exe_path	path of this executable, if any
System.env(name) / env_keys	environment variables; env returns Option<string>, so use unwrap_or(fallback) for a default and is_some() to test presence
System.set_env(name, value) / unset_env(name)	set or clear an env var
System.cwd / set_cwd(path)	working directory
System.temp_dir / home_dir	temp and home paths
System.cpu_time_nanos	process user+system CPU time since start
System.memory_bytes	OS resident memory (RSS / working set); 0 if unknown

Math (no import)

Example: Math.sqrt(9.0) → 3.0. A negative input is NaN.

Math.floor, ceil, round, sqrt, pow, sin / cos / tan, asin / acos / atan / atan2, log / log10 / exp / hypot, and Math.PI / Math.E. Angles are radians. round sends halfway cases away from zero (Math.round(-2.5) is -3.0). Every function gives the same result on native and WebAssembly targets.

Absolute value, minimum, maximum, and clamping are methods on the number itself: x.abs(), x.min(y), x.max(y), and x.clamp(low, high).

Timing

Time.delay(ms).await / Time.sleep(ms).await need an async function. Time.nano_time() is a monotonic clock. Stopwatch times a span — also on DateTime.

Other

• .length on strings, arrays, and collections.
• Array.alloc<T>(n) — a zeroed fixed array. Prefer List when the size grows.
• Override public override fun to_string() so print and + show your type. See object.

Waiting for cancellation

Cancellation tokens can suspend until cancellation, or wait for another future while observing cancellation. Waits use scheduler notifications rather than periodic checks. wait returns Err(CancelledError) and cancels the supplied future when cancellation wins; completed side effects remain observable. A wait detaches its subscription on completion or when its cancelled future is released. Tokens retain their existing cross-worker sharing behavior. Use wait_void for a Future<void>; it returns Ok(Unit()) when the operation completes.

import system;

async fun cancellation_value(): string {
    Time.delay(0).await;
    return "ready";
}

async fun main(): void {
    let source = CancellationSource();
    System.println(source.token.wait(cancellation_value()).await.unwrap());
    source.cancel();
    System.println(source.token.wait_cancelled().await);
}