如何解决这种类型不匹配？

Question

假设我这样定义zipWith：

def zipWith[A](f:(A, A) => A)(xs:Iterable[A], ys:Iterable[A]): Iterable[A] =
  (xs, ys).zipped map f

现在我想使用这样的压缩矩阵：

type Matrix = Vector[Vector[Int]]

val zipMatrix: ((Int, Int) => Int) => Matrix => Matrix => Matrix = f =>
  zipWith(zipWith(f)) 

但是我得到了一个错误：

<console>:15: error: type mismatch;
found   : (Iterable[Iterable[Int]], Iterable[Iterable[Int]]) => Iterable[Iterable[Int]]
required: Matrix => (Matrix => Matrix)
(which expands to)  scala.collection.immutable.Vector[Vector[Int]] => (scala.collection.immutable.Vector[Vector[Int]] => scala.collection.immutable.Vector[Vector[Int]])

为什么 Vector[Vector[Int]] 不匹配 Iterable[Iterable[Int]] ？如何解决这个错误？

Answer 1

考虑一下：

def zipWith[A] (f:(A => A => A))(xs:Vector[A])(ys:Vector[A]): Vector[A] =
  (xs, ys).zipped.map {case (x, y) => f (x)(y)}

val zipMatrix: (Int => Int => Int) => (Matrix => Matrix => Matrix) = f =>
  zipWith(zipWith(f))