class Vector3 {
  constructor(public x: number, public y: number, public z: number);
}

const field: ReversibleTriFunction<number, Vector3> = {
  forward: (x, y, z) => new Vector3(2*x , 2*y, 2*z),
  backward: r => [0.5 * r.x, 0.5 * r.y, 0.5 * r.z],
}

const r = field.forward(2, 1, 4); // => Vector3(4, 2, 8)
field.backward(r); // => [2, 1, 4]

typeparam

Type of the function return value.