What does "T =:= Int = null" mean in Scala?

user_1031769 0
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As in the following code.

class A[T] {
  def add(n: Int)(implicit env: T =:= Int = null): Int = n + 1
}

object A extends App {
  val a = new A[Int]
  a.add(1) // 2
}

I know that T =:= Int means T should be of type Int, but what does = null part mean?

Note: The example is made up by me. It'd be better if you can show me a proper usage of = null if it's not appropriate.

scala
1 Answers

null is just assigning a default value to the ev, just like you would to any other parameter. It is a clever way to find out whether the type is actually an Int:

 def isInt[T](implicit ev: T =:= Int = null): Boolean = env != null

 isInt[Int] // true
 isInt[String] // false

The trick is that when compiler sees the Int, it will pass in the actual implicit value, and when it can't find one, it'll just leave it as default. So, by checking if ev is null, you can tell whether or not the implicit was available at call site.

user_42545170
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