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

In addition to separating the parameter list from the function body in a function literal, the double arrow can be used as syntactic sugar for a FunctionN type:

T => R means Function1[T, R]

(T1, T2) => R means Function2[T1, T2, R]

...

In your example, this means that op is a function that takes a Double and returns a Double (as the author explained).

As another example, it's possible to write

// declare len as a function that takes a String and returns an Int,
// and define it as returning the length of its argument
val len: String => Int = { _.length }

// or, a more verbose version that uses '=>' in both ways
val len: String => Int = { (s: String) => s.length }

len("apple") // returns 5