[swift-evolution] Localization support for string interpolation

Dave Abrahams dabrahams at apple.com
Mon Apr 25 17:03:22 CDT 2016

on Sun Apr 24 2016, Chris Lattner <swift-evolution at swift.org> wrote:

>> On Apr 24, 2016, at 2:33 PM, Chris Lattner via swift-evolution
> <swift-evolution at swift.org> wrote:
>>> On Apr 21, 2016, at 12:42 AM, Daniel Höpfl via swift-evolution
> <swift-evolution at swift.org> wrote:
>>> Hi there!
>>> TL;DR: Here’s my idea for a better localized strings handling in Swift.
>>> It includes both, support for string interpolation support for
>>> NSLocalizedString, and a new string delimiter `...` that can be
> used
>>> instead of NSLocalizedString("...”).
>> FWIW, this is closely related to the idea of extending string
>> interpolation to support generalized “printf” style modifiers, which
>> would allow very expressive formatting inline in a string
>> interpolation.  This has not yet come to pass, but a write up of the
>> ideas are available here:
>> https://github.com/apple/swift/blob/master/docs/TextFormatting.rst
> Actually, it looks like the interesting point didn’t make it into the
> formal writeup.  The idea was to enable something like this (just a
> sketch):
> public protocol CustomStringConvertible {
>   /// A textual representation of `self`.
>   var description: String { get }
>   func formattedDescription(format: String) -> String { get }
> }
> Int could then implement support for “x” to print hexadecimal, we
> could support left/right whitespace padding, and custom types could
> provide their own custom formatters (e.g. a string could have a title
> case formatter, or a "look up localized form of” modifier).  We could
> then provide a “printf” that would allow traditional “%x0”
> substitutions.  It could also be incorporated into the string literal
> syntax by allowing something like:
> 	let hashValue = 42  // not a good hash function
> 	print(“Hashcode = \(hashValue:x)”)
> This is just a sketch, but that’s the idea.  It never went anywhere
> because we didn’t have the language features in place to make it work,
> notably defaulted implementations in protocols were missing at the
> time.

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.


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