Right Arrow meanings in Scala

Question

In Chapter 9 of Programming In Scala, there is an example method like this:

def twice(op: Double => Double, x: Double) = op(op(x))

The

Answer 1

That is a function that takes a double and returns a double.

scala> def  d2d = (d:Double) => d 
d2d: (Double) => Double

It's type is Function1[Double,Double]