[Pre-RFC] Safe goto with value

language design
56

I’ll note that I did, at one point, contemplate having something like

fn foo<T, 'a: break(T)>(val: T) -> ! {
    break 'a val;
}

fn bar() {
    let x = 'a: {
        foo<'a>(5)
    };
    println!("{}", x); // 5
}

Which is why I chose fsm! for my version of it, as after all one of things it stands for is ‘finite-state machine’. Here’s your example translated:

let result = fsm! s1(42) {
	s1(n: i32) => {
		if n % 2 == 0 {
			continue s3(n / 2, n + 1)
		}
		s2
	},
	s2 => s1(84),
	s3(x: i32, y: i32) => match foo {
		None => break x + y,
		Some(n) => continue s1(n)
	},
};

No new keywords necessary, and it can be easily read from the top, just like a match expression. I think it could be implemented on top of loop and match as a procedural macro. A simplified version (without continue and with slightly different initial state syntax) is easily implementable with a pattern-match macro.

(Aside: the funclets proposal for WebAssembly seems related, although it is a bit more general: by the look of it, it should be able to express things like binary tree traversal, unlike fsm!.)

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