faking ADTs in TypeScript

adt.ts

type Meters = { meters: number };
	type Feet = { feet: number };
	type Distance = Meters | Feet;
	function isMeters(distance: Distance): distance is Meters {
		return typeof (distance as Meters).meters === 'number';
	}
	function isFeet(distance: Distance): distance is Feet {
		return typeof (distance as Feet).feet === 'number';
	}

function checkDistance(x: Distance): string {
		if (isMeters(x)) {
			return "meters";
		} else if (isFeet(x)) {
			return "feet";
		}
		return "neither";
	}