Variance modifiers are one of those parts of Kotlin that all use (often unconsciously), but only a small portion of developers understand it well. It gives us the convenience of using collection or functional types and lets us implement popular patterns, like Covariant Nothing Object (used by emptyList or many data structures) or Covariant Nothing Class (used by Either and on many implementations of Result). I will deeply explore those concepts and patterns in this presentation, concentrating on practical use cases.

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