Scala map with partial function as value - scala

Question

In Twitter's Scala school collections section, they show a Map with a partial function as a value:
// timesTwo() was defined earlier.
def timesTwo(i: Int): Int = i * 2
Map("timesTwo" -> timesTwo(_))
If I try to compile this with Scala 2.9.1 and sbt I get the following:
[error] ... missing parameter type for expanded function ((x$1) => "timesTwo".$minus$greater(timesTwo(x$1)))
[error] Map("timesTwo" -> timesTwo(_))
[error] ^
[error] one error found
If I add the parameter type:
Map("timesTwo" -> timesTwo(_: Int))
I then get the following compiler error:
[error] ... type mismatch;
[error] found : Int => (java.lang.String, Int)
[error] required: (?, ?)
[error] Map("timesTwo" -> timesTwo(_: Int))
[error] ^
[error] one error found
I'm stumped. What am I missing?

Answer 1

It thinks you want to do this:
Map((x: Int) => "timesTwo".->timesTwo(x))
When you want this:
Map("timesTwo" -> { (x: Int) => timesTwo(x) })
So this works:
Map( ("timesTwo", timesTwo(_)) )
Map("timesTwo" -> { timesTwo(_) })
Note this is not an usual error, see
https://stackoverflow.com/a/7695459/257449.
Scala underscore - ERROR: missing parameter type for expanded function
(and probably more)

Answer 2

You are missing telling scalac that you want to lift the method timesTwo into a function. This can be done with an underscore as follows
scala> Map("timesTwo" -> timesTwo _)
res0: scala.collection.immutable.Map[java.lang.String,Int => Int] = Map(timesTwo -> <function1>)