Scala中>：的目的或意义是什么

Question

Such as List.contains ,here is the source code from scala api.

 def contains[A1 >: A](elem: A1): Boolean = {
   var these = this
   while (!these.isEmpty) {
     if (these.head == elem) return true
     these = these.tail
   }
   false

I understand the inter code theory, But what's about type A1 >: A ?

I guess >: just like isInstanceOf or something like to limit the input param's type?

Could some one give a concise explanation or some docs so i can get some research

Answer 1

The meaning:

A1 is the nearest common ancestor of A and supplied argument type.

The purpose:

since List is declared as List[+A] where +A means "covariant on type A", using A as an argument type is not allowed:

scala> :pa
// Entering paste mode (ctrl-D to finish)

class List[+A] {
  def m(x: A) = x
}

// Exiting paste mode, now interpreting.

<console>:15: error: covariant type A occurs in contravariant position in type A of value x
         def m(x: A) = x

UPDATE

Naive explanation why not (just a guess, it's hard to prove since compiler won't allow it):

class List[+A] {
  def m(x: A) = x
}

class Fruit
class Apple extends Fruit
class Pear extends Fruit

// list is covariant, which means
// List[Fruit] is a supertype of List[Apple]
val l: List[Fruit] = new List[Apple]
// i.e. l.m has to accept Fruit
l.m(new Pear) // Apple?

in reality:

class List[+A] {
  def m[A1 >: A](x: A1) = x
}

...

l.m(new Pear) // Fruit, the nearest common ancestor of Apple and Pear

Answer 2

A >: B表示B是A的下限类型