[swift-evolution] Localization support for string interpolation

[Uli Kusterer]

On 24 Apr 2016, at 23:40, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 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.

 I'm not quite sure why we'd want to support printf-style syntax in the first place? Interpolation is more readable, as the formats contain their descriptive variable names, and you don't have to worry about matching up format string placeholder order and types with those of what you're actually passing. I.e. string interpolation is to printf what type inference is to explicit declarations, and what labeled parameters are to old-style C initializer lists.

 So I'd say Swift should come at this from the other direction and ensure that anything you were able to do with printf is also possible with interpolation (if that means specifying some sort of format flag in-line as well, I could live with that).

 I do see the utility in allowing any custom type to be printed out via interpolation though, and to allow them to at the least query the existing flags as well. But should we really use single-character flags? C++, apart from overloading bitshift, has the right idea:

	cout << "foo" << hex << 123 << "\n";

is much more readable than

	printf( "foo%x\n", 123 );

Maybe we could just use a function as the flag?

	print( "foo\(hex(123))\n" )

would then expand into "foo" hex(123) "\n", where hex would just be an object or function that takes an Int and returns a hex-formatted string? Though this'd pollute the namespace I suppose. 123.hex() maybe? Then it could be an arbitrary method on Int, added with a category.

Cheers,
-- Uli Kusterer
"The Witnesses of TeachText are everywhere..."
http://orangejuiceliberationfront.com