What does [A=>B] mean in a type constructor in scala

Question

Given some simple class Container with a type parameter T

case class Container[T](value:T)

Now defining a method

def test[A,B](b:Container[A=>B]) = {

}

What does this [A=>B] mean ? Is it a bound or some kinde of a function type ?

Answer 1

It is a function. It means that T is a function that takes type A as an input and returns a value of type B.

For example,

val c = Container[String=>Int]( s => s.length() )
c.value("abc")   // returns 3, and is the equivalent to c.value.apply("abc")
                 // which in turn calls the function s => s.length()
                 // and so returns "abc".length()

Answer 2

The type [A=>B] is a function that takes inputs of type A and produces outputs of type B .

In the context of a constructor, it means you are passing in a function as a parameter to the constructor.

Answer 3

A side note, for instance a function value

val fv = (v: Int) => v + v

creates a function object instance at runtime and extends Function1 trait, which includes an apply method, as aforementioned. Equivalently,

val fv = new Function1[Int, Int] {
  def apply(v: Int) = v + v
}