Contravariance: intuition building from first principles
summary
Builds intuition from first principles by showing exact substitutability rules, changing a type parameter’s variance to make code compile, and demonstrating which direction substitution must go. Works through a concrete Animal hierarchy with Rescue/Clinic typeclasses to show why Rescue is covariant (+) and Clinic contravariant (−), then tests the idea with JsonEncoder/JsonDecoder and real Scala compiler behavior, and points to common real-world occurrences (codecs, Show, function inputs).
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chapters (7)
- 0:00Introduction and motivation
- 2:56Prerequisites
- 4:52Covariance and subtyping
- 12:26Understanding contravariance
- 17:04Practical use cases
- 23:46Why contravariance is hard
- 29:17Conclusion and Q&A
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