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You can prevent external packages from implementing your interfaces by adding unexported method names to your interface. Usually I'd call it an anti-pattern for interfaces, but I've done it a couple of times when there was literally no conceivable way for an external package to implement something conforming to a particular interface for various technical reasons.

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Yeah, most "good" uses of this technique are kindof abusing it to substitue for some other feature the language doesn't have, the most common of which I can think of is combining it with type switches to get a poor-man's sum types (what rust calls enums). You see this a lot in code working with ASTs, especially.

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