What does "code: => Unit" mean in scala?

Question

Does anyone know the type of => Unit in scala? I don't know the meaning of => Unit and how to use it. I defined a function like below:

def test(code: => Unit){    print("start ...")    code    print("end ....") }  test(print(1)) 

Does it means a function with any arguments returning Unit?

Thanks

Answer 1

This kind of parameter are called by-name parameter

=> B represents a block a of code which return a B value, their purpose is that they are evaluated only when you call the parameter.

def foo(code: => Int) {     println("Not yet evaluated")     val result = code     println("parameter evaluated %s, is it an int ? %s " format (            result, result.isInstanceOf[Int]) ) } 

And you can call foo in the following way :

foo(1)  

or

val a = 3 val b = 5 foo {   val c = a * a   c * b } 

The other style of passing parameter is by-value : parameters are evaluated before they are sent to the method

def foo(code : Int) {     println("Parameter already evaluated")     val result = code     println("parameter evaluated : " + result) } 

References

Extract from the Book Functionnal Programming in Scala

More differences between by-name parameter and by-value parameter illustrated