leviathan-lang.com / docs / learn

Errors and resources

Expected absence as a value, programmer errors as exceptions, and deterministic cleanup with using.

Absence versus errors

Leviathan draws a deliberate line. Expected absence is a value; programmer errors throw. A parse that might fail returns a T?; an out-of-bounds index throws a catchable RuntimeException.

Optionals for parsing, exceptions for bugs

int? parsed = "42".toInt();      // strict: None on garbage, never a silent 0
int n = parsed ?? 0;
console.writeln(n);
int bad = "forty-two".toInt() ?? -1;
console.writeln(bad);

// Throwing path: the thrown value must implement IException.
try {
    throw RuntimeException("boom");
} catch (IException e) {
    console.writeln("caught: ${e.message}");
}

Array<int> xs = [1, 2, 3];
try {
    console.writeln(xs.at(10));
} catch (RuntimeException e) {
    console.writeln("caught: ${e.message}");
}

Run it ▶

42
-1
caught: boom
caught: index 10 out of bounds (length 3)

Deterministic cleanup

There is no finally. Deterministic cleanup is using: it owns a resource that implements IDisposable and runs close() on every way the block exits: falling off the end, return, a throw unwinding past it, or a break. Multiple resources close in reverse order.

using closes in reverse order

class Resource : IDisposable {
    public string name;
    public void close() { console.writeln("closed ${name}"); }
}

void work() {
    using Resource a = Resource("first");
    using Resource b = Resource("second");
    console.writeln("working");
}   // b.close() runs, then a.close(), on every exit edge

work();

Run it ▶

working
closed second
closed first

The same rule makes files safe. File implements IDisposable, so both files below are closed however backup exits:

Copying a file

void backup(string path) {
    using File src = File(path, std::read);
    using File dst = File(path + ".bak", std::write);
    dst.writeln(src.readln());
}   // dst.close() runs, then src.close(), on every exit edge

not run — reads and writes files on disk