CODEWITHSUNDEEP
scalascalazapplicative
Michael
Michael

Reputation: 42050

Applicative instance for ZipList in Scala

This is a follow-up to one of my recent previous questions:

I would like to define a zip Applicative instance for List (and probably Set and Map). For example:

val xs: List[Int] = List(1, 2, 3)
val fs: List[Int => Int] = List(f1, f2, f3)
val ys: List[Int] = xs <*> fs // expected to be List(f1(1), f2(2), f3(3))

So I defined a ZipList and its Applicative:

case class ZipList[A](val list: List[A])

implicit val zipListApplicative = new Applicative[ZipList] {

  def point[A](a: => A): ZipList[A] = ZipList(List(a))

  def ap[A, B](za: => ZipList[A])(zf: => ZipList[A => B]): ZipList[B] = {
    val bs = (za.list zip zf.list) map {case (a, f) => f(a)}
    ZipList(bs)
  }
}

and can use it as follows:

 scala> val xs: List[Int] = List(1, 2, 3)
 xs: List[Int] = List(1, 2, 3)

 scala> val fs: List[Int => Int] = List(_ + 2, _ + 2, _ +1)
 fs: List[Int => Int] = List(<function1>, <function1>, <function1>)

 scala> ZipList(xs) <*> ZipList(fs)
 res4: ZipList[Int] = ZipList(List(3, 4, 4))

It seems to be working but maybe I am missing something.

Upvotes: 1

Views: 186

Answers (1)

Lee
Lee

Reputation: 144136

Applicatives should satisfy the law

point identity <*> v == v

which yours does not since

point identity List(1,2,3) == List(1)

pure a for a zip list should return an infinite stream of a which is why you need a lazy data structure.

Upvotes: 4

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