How to define scala type for no argument function?

Question

This works

def func(f: => Int) = f

This dosn't (inside class for example)

type EmptyFunct = => Int

or

 type EmptyFunct = (=> Int)

Scala version 2.9 Two questions:

1. Why dosn't syntax sugar work in second case?
2. How to define this function without syntax sugar?

Sorption

Can you give an example of the usage of EmptyFunct that shows why () => Unit doesn't fit your need? – Annecy

Answer 1

=> Int is not exactly a function without parameter, it is an Int argument with call by name passing convention. (Of course, that is rather fine a point, as it is implemented by passing a function without parameter ).

The function without argument is written () => Int. You can do type EmptyFunct = () => Int.

It is not a type. Inside func, f will be typed as an Int. An argument of type () => Int will not.

def func(f: => Int) = f *2

func (: => Int) Int

But

def func(f: () => Int) : Int = f*2

error: value * is not a member of () => Int

Nianiabi

So there are no way to create collection of no arg (: =>Unit) functions? – Sorption

See my response: Collection[Function0[Unit]] – Annecy

If you were looking for (: => Unit *) repeated arguments, it is not allowed. Indeed, that could only be Seq[Unit] (Seq contains values, not arguments calling conventions). And be quite useless. What you want is Seq[Function0[Unit]] (or Seq[() => Unit], it means the same thing). Notation will be less convenient : () => x += 1 instead of x+= 1. – Nianiabi

To define a collection: val c: List[()=>Unit] = List( () => println("hello"), () => println("world") ). – Humism

Answer 2

You should use Function0

In the first case it does not work because you declare a non argument Function but because you indicates that the parameter is called by name.

Annecy

This is not solution to my problem. Because Function0 is actually has type () => T not (=> T) which as I undertand not function at all. – Sorption

Answer 3

I don't see much sense in a method, returning an int, invoked with no parameter. Either it returns a constant, so you could use the constant, or it would use a var?

So let's go with a var:

var k = 10 
val fi = List (() => k * 2, () => k - 2)
val n = fi(0)
n.apply 
k = 11
n.apply

result is 20, then 22.

Mariettamariette

"I don't see much sense in a method, returning an int, invoked with no parameter." A counter ? – Annecy

Or any callback function which has all its context in creation phase – Sorption