The diamond of variance
summary
Argues that Swift's generics map onto a four-point "diamond of variance" and walks through concrete Swift code snippets showing covariance, contravariance, invariance, phantom as distinct cases. Demonstrates Optionals and closures as examples (optionals behaving like covariance, function parameters like contravariance), and shows tiny toy types where an unused generic parameter produces a phantom type. Claim: these examples expose how Swift's type system fits the theory rather than just language folklore.
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chapters (6)
- 0:00Introduction
- 0:48Understanding covariance
- 1:25Covariance in practice
- 2:20Contravariance explained
- 3:22Invariance and phantom types
- 4:21Recap and conclusion
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