Opaque types vs Existential Types vs Generics

▤ Summary

This article explains the differences between opaque types, existential types, and generics in Swift. It shows how `some Protocol` hides implementation details while preserving concrete type identity, and how `any Protocol` stores values through an existential container. It also covers the runtime costs and dispatch behavior of existential values, including inline storage, witness tables, and possible heap allocation. The article ends by showing when associated types force you back to generics with same-type constraints. The main takeaway is that you should choose the abstraction that matches your need for type identity, flexibility, and performance.

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