TensorDims.swift

protocol Tensor {
  static var numberOfDims: Int { get }
}

struct NestedTensor<Element: Tensor>: Tensor {
  static var numberOfDims: Int {
    return 1 + Element.numberOfDims
  }
  
  // ...etc...
}

struct UnitTensor<Unit>: Tensor {
  static var numberOfDims: Int {
    return 0
  }
  
  // ...etc...
}

let x = NestedTensor<NestedTensor<UnitTensor<Int>>>()

x.dynamicType.numberOfDims  // returns 2