In and out type variant of Kotlin

If you ever defined generic in Kotlin, you’ll notice many a times, it would propose to use the in or out keyword to define the generic. It puzzles me at a start on when which is used, and for what.

Formally, this is a way to define contravariance and covariant. It took me a while to learn about it. I’ll dive in here to explain what how I understand and memorize their different.