Bug? Memoized function doesn't compile inside closure

Question
1

I get “Error: forward reference extends over definition of value bar” for this code

case class Memo[A,B](f: A => B) extends (A => B) {
	val hashTable = scala.collection.mutable.Map.empty[A,B]
	def apply(x: A) = hashTable.getOrElseUpdate(x, f(x)) 
}

def foo = {
	val bar: Memo[Int, Int] = Memo {n => bar(n-1)} // Error here
	bar(0)
}

The code compiles if I move bar out of foo. I use Scala IDE 4.6 with Scala 2.12.2.

2

This behavior is completely expected and consistent with the scoping rules of the language. You are using bar in its declaration. This won’t work for a local variable as you have in this code. The reason it compiles if you move it outside of foo is that it becomes a class-level member, where you have different scoping rules. Members of classes are in scope before the point of their declaration while local variables aren’t. However, if a member is referenced before the execution reaches the point of initialization, it will have a default value, which in the case of an AnyRef, is null.

3

So that means that values can be recursive only if they are class members? Which is different from functions defined with def.

4

I don’t really know if I have a complete explanation. You can have a recursive val in a method if it is declaring a function. For example:

val fact: Int => Int = n => if (n<2) 1 else n*fact(n-1)

This is perfectly happy inside of a method. I don’t know if this is a special case in the compiler. In theory, what you are doing could work similarly, because bar is only referenced inside of a lambda expression, but the compiler might have a harder time recognizing that.

Your code compiles if bar is a def declaration instead of a val declaration, but that produces very different code that isn’t really doing what you want.