Scala underScore strange behavior: error: missing parameter type for expanded function

Question

First, this:

"1 2".split(" ").toSet

and this:

Set("1", "2")

both evaluate to the same thing, namely

res1: scala.collection.immutable.Set[String] = Set(1, 2)

Why then, when I do:

Set("1", "2") map (_.toInt)

I get as expected this:

res2: scala.collection.immutable.Set[Int] = Set(1, 2)

but when I do this:

"1 2".split(" ").toSet map (_.toInt)

I got:

<console>:12: error: missing parameter type for expanded function ((x$1) => x$1.toInt)
   "1 2".split(" ").toSet map (_.toInt)

I checked and additional parentheses do not solve the problem.

Answer 1

The code should be:

"1 2".split(" ").toSet map (x: String => x.toInt)

Here, I am explicitly specifying that the Set contains strings.

Chain calls have this issue in Scala where the compiler expects you to provide the type of the parameter.

Answer 2

The reason in in type inference when using toSet so you need to have a type hint for chain calls or split the calls. You can find details here https://issues.scala-lang.org/browse/SI-7743, https://issues.scala-lang.org/browse/SI-9091