I am trying to use the fold function on Lists that contain elements of different types. Here's a simplified version of my code:

val list: List[Int] = List(1, 1, 1, 1)
val listOfMap: List[Map[String, Int]]= List(Map("a" -> 1), Map("a" -> 2))

def addN(list: List[Int], n: Int) = list.map(_ n)

val result = listOfMap.fold(list)((x, y) => addN(x, y("a")))

I am expecting the result will be a List of Integer:

List(4, 4, 4, 4)

However, the error that I got were Type Mismatch for:

x : Required: List[Int], Found: Iterable[Any] with Partial Function[Int with String, Int] with Equals

"a": Required: Int with String, Found String

CodePudding user response：

fold don't do what you think. For this kind of problem, Scala standard Library Documentation is your friend : https://www.scala-lang.org/api/2.13.3/index.html

def fold[A1 >: A](z: A1)(op: (A1, A1) => A1): A1

Folds the elements of this collection using the specified associative binary operator.

You need foldLeft instead

def foldLeft[B](z: B)(op: (B, A) => B): B

Applies a binary operator to a start value and all elements of this sequence, going left to right.

Note: will not terminate for infinite-sized collections.

in your case

val result = listOfMap.foldLeft(list)((x, y) => addN(x, y("a")))
//List(4, 4, 4, 4): scala.collection.immutable.List[scala.Int]