Arbitrary numeric tensors in TypeScript

vec.ts

type Array_Element<A> = A extends (infer T)[] ? T : never

type Fixed_Array<A, X> =
  [A] extends [[]] ? [] :
  [A] extends [[X, ...infer T]] ? [X, ...Fixed_Array<T, X>] : never

type Vector<V> = Fixed_Array<V, Vector_Car<V>>
type Vector_Car<V> = [V] extends [number[]] ? number : Vector<Array_Element<V>>

class Vec<V extends Vector<V>> {
  constructor(public readonly _: V) { }

  plus(v: V): Vec<V> {
    return new Vec(this._.map((x, i) =>
      typeof x === 'number' ? x + v[i] :
        new Vec(x).plus(v[i])
    ) as V)
  }
}

function vec<V extends Vector<V>>(x: V) {
  return new Vec(x)
}

console.log(vec([1, 2, 3]).plus([6, 4, 2])._)