[swift-evolution] StringProtocol - Do not declare new conformances?

Michael Ilseman milseman at apple.com
Tue Sep 19 18:38:02 CDT 2017

Out of curiosity, how are your other string types different from Swift String?

> On Sep 19, 2017, at 3:34 PM, Braden Scothern via swift-evolution <swift-evolution at swift.org> wrote:
> Where I work we are writing a C library for cross platform development on iOS, Android, and Ubuntu. Part of what I do with this library is wrap it in Swift. We have a string type that should conform to StringProtocol. The issue is that the StringProtocol has this statement for its documentation overview:
> "Do not declare new conformances to StringProtocol. Only the String and Substring types of the standard library are valid conforming types."
> While I don't think many people will/should conform to StringProtocol, I feel like discouraging the usage of the protocol in the rare cases is is applicable takes away a very powerful tool from the language.
> Are there specific implementation details in the standard library that make it so other types cannot safely conform to StringProtocol? If not is there a compelling reason for this statement to be on the protocol?
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