flatMap strange behaviour on List of List of Integers

Question

In Scala:

scala> val xs = List(List(1, 3, 5), List(3, 4, 30))
xs: List[List[Int]] = List(List(1, 3, 5), List(3, 4, 30))

scala> xs flatMap {x => x + 1}
<console>:9: error: type mismatch;
 found   : Int(1)
 required: String
              xs flatMap {x => x + 1}

Why?

score 2 · Answer 1 · answered Sep 11 '15 at 10:35

The simplest solution is to flat your list before executing map:

xs.flatten.map { _ + 1 }

The reason for your error is that flatMap doesn't flatten the collection on which you execute it, it flattens the results returning by the function in its argument.

score 2 · Answer 2 · answered Sep 11 '15 at 10:36

2

String in that error message is an unfortunately misleading inference.

The error the compiler ideally would have given you is

found   : Int => Int
required: Int => List[Int]

answered Sep 11 '15 at 10:36

Chris Martin

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I accept your answer because this was what I am asking about – Muhammad Hewedy Sep 11 '15 at 10:38

score 1 · Answer 3 · answered Sep 11 '15 at 10:34

1

x in the flatMap is a List[Int] not an Int, two solutions come to mind:

scala> xs.flatMap(identity).map(_ + 1)
res2: List[Int] = List(2, 4, 6, 4, 5, 31)

scala> xs.flatMap(_.map(_ + 1))
res3: List[Int] = List(2, 4, 6, 4, 5, 31)

answered Sep 11 '15 at 10:34

Ende Neu

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You can use `flatten` instead of `flatMap(identity)`. A for expression is also a good fit here. – marstran Sep 11 '15 at 10:35

flatMap strange behaviour on List of List of Integers

3 Answers3