[swift-evolution] Localization support for string interpolation

[Brent Royal-Gordon]

> I believe I also had concerns about this design because it needlessly
> misses opportunities for compile-time type safety.  There doesn't seem
> to be any reason to embed the format specifier in a dynamic string that
> has to be parsed at runtime.

I started drafting a post yesterday making the same point, but got bogged down in writing an example of what I *would* want to see.

In brief: printf()-style specifiers are kind of awful. They're very cryptic and unnecessarily unsafe. In C, this problem is addressed by compiler warnings, but Swift already has a mechanism to detect inappropriate combinations of data and operations: the type system. We should use it here.

If we had a protocol to handle formatting, I would want the method's parameter to be of an associated type and typically be something OptionSet-like. It might even be better to structure it as some sort of Formatter protocol. If properly designed, this might allow us to leverage Foundation's existing, fully-featured formatters rather than writing new ones just for Swift.

(This would be a big new feature, so personally, I would defer it.)

-- 
Brent Royal-Gordon
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