Written version: https://blog.rockthejvm.com/scalatyp...

Links from the video:
Why is contravariance so hard    • Contravariance in Scala: Why Is It So...  

The advanced Scala course that discusses lots of these type system issues including variance: https://rockthejvm.com/p/advancedscala'>https://rockthejvm.com/p/advancedscala

This video is for the mature Scala developer. We talk about variance in Scala as an innocent question: if A extends B, should Thing[A] be a subtype of Thing[B]?

We then show the variance possibilities in Scala, and we explore the kind of problems this can pose. The infamous and cryptic "covariant type T occurs in contravariant position" will come up and we'll prove why that is the case.

Finally, we'll solve the "covariant type in contravariant position" problem with some generic type magic.

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