scala - Making sense of forall and exists output on empty list

Question

scala> val l = List()
l: List[Nothing] = List()

scala> l.forall(x=>false)
res0: Boolean = true

scala> l.forall(x=>true)
res1: Boolean = true

scala> l.exists(x=>false)
res2: Boolean = false

scala> l.exists(x=>true)
res3: Boolean = false

For above 2 predicate, now that no element exists in the list, how come forall return true? I am confused. could you somebody help explain?

Answer 1

You could rephrase that forall means that none of the elements of the list violate the given predicate. In case there are no elements, none of them violates it.

The source code of forall explicitly returns true if a collection is empty:

def forall(p: A => Boolean): Boolean = {
    var these = this
    while (!these.isEmpty) {
       ...
    }
    true
}