Question

What does [B >: A] mean in Scala? And what are the effects?

Example reference: http://www.scala-lang.org/node/129

class Stack[+A] {
    def push[B >: A](elem: B): Stack[B] = new Stack[B] {
        override def top: B = elem
        override def pop: Stack[B] = Stack.this
        override def toString() = elem.toString() + " " + Stack.this.toString()
    }
    def top: A = error("no element on stack")
    def pop: Stack[A] = error("no element on stack")
    override def toString() = ""
}

object VariancesTest extends Application {
    var s: Stack[Any] = new Stack().push("hello");
    s = s.push(new Object())
    s = s.push(7)
    println(s)
}

Answer 1

[B >: A] is a lower type bound. It means that B is constrained to be a supertype of A.

Similarly [B <: A] is an upper type bound, meaning that B is constrained to be a subtype of A.

In the example you’ve shown, you’re allowed to push an element of type B onto a stack containing A elements, but the result is a stack of B elements.

The page where you saw this actually has a link to another page about lower type bounds, which includes an example showing the effect.