[swift-evolution] Strings in Swift 4

Gwendal Roué gwendal.roue at gmail.com
Mon Jan 23 22:29:08 CST 2017

> Le 24 janv. 2017 à 04:31, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> a écrit :
>>> The operands and sense of the comparison are kind of lost in all this garbage. You really want to see `foo < bar` in this code somewhere, but you don't.
>> Yeah, we thought about trying to build a DSL for that, but failed.  I think the best possible option would be something like:
>>  foo.comparison(case: .insensitive, locale: .current) < bar
>> The biggest problem is that you can build things like
>>    fu = foo.comparison(case: .insensitive, locale: .current)
>>    br = bar.comparison(case: .sensitive)
>>    fu < br // what does this mean?
>> We could even prevent such nonsense from compiling, but the cost in library API surface area is quite large.
> Is it? I think we're talking, for each category of operation that can be localized like this:
> * One type to carry an operand and its options.
> * One method to construct this type.
> * One alternate version of each operator which accepts an operand+options parameter. (I'm thinking it should always be the right-hand side, so the long stuff ends up at the end; Larry Wall noted this follows an "end-weight principle" in natural languages.)
> I suspect that most solutions will at least require some sort of overload on the comparison operators, so this may be as parsimonious as we can get. 

SQL has the `collate` keyword:

	-- sort users by email, case insensitive
	select * from users order by email collate nocase
	-- look for a specific email, in a case insensitive way
	select * from users where email = 'foo at example.com <mailto:foo at example.com>' collate nocase

It is used as a decorator that modifies an existing sql snippet (a sort descriptor first, and a comparison last)

When designing an SQL building to Swift, I chose the `nameColumn.collating(.nocase)` approach, because it allowed a common Swift syntax for both use cases:

	// sort users by email, case insensitive
	// look for a specific email, in a case insensitive way
	User.filter(nameColumn.collating(.nocase) == "foo at example.com <mailto:foo at example.com>")

Yes, it comes with extra operators so that nonsensical comparison are avoided.

But it just works.


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