Scala forward reference of nested recursive function

Question

I have this really simple method definition with a nested recursive function:

def bar(arr : Array[Int]) : Int = {
  val foo : Int => Int = (i: Int) => if(i == 0) 0 else i + foo(i-1)
  foo(3)
}

But I get this error:

<console>:36: error: forward reference extends over definition of value foo
     val foo : Int => Int = (i: Int) => if(i == 0) 0 else i + foo(i-1)
                                                              ^

If I just put the val foo: ... = ... line by itself, and not nested within a def, everything works

Answer 1

You can make it a lazy val:

def bar(arr : Array[Int]) : Int = {
  lazy val foo : Int => Int = (i: Int) => if(i == 0) 0 else i + foo(i-1)
  foo(3)
}

or a def:

def bar(arr : Array[Int]) : Int = {
  def foo(i: Int): Int = if(i == 0) 0 else i + foo(i-1)
  foo(3)
}

When you

put the val foo: ... = ... line by itself, and not nested within a def

it becomes a combination of a field and a getter method, and foo(i-1) actually calls the getter method instead of referring to the value you are defining, which is illegal; but when you have a val inside a method, it's just a local variable and doesn't have a getter method.