Controlling Access Within a Swift Package

TL;DR: Swift 5.9 introduces the package access modifier, allowing APIs to be shared across Targets within the same package without exposing them publicly. This supports safer and more modular Swift code by enabling internal sharing while preserving external encapsulation—ideal for designs like multi-Target packages that require internal-only protocols or methods.

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Background

With the rise of modular programming, developers are increasingly using Swift Package Manager (SPM) to separate functionalities into multiple packages. Within a single package, it’s common to further divide code into multiple Targets. A recurring challenge in this structure is…