[ios, macos] Write "collator" wrappers for NSExpression

[ChrisLoer]

Follow-on for #11869: ios/macos now support "collator" expressions, but they're not directly exposed via the SDK. Collator expressions take a dictionary-style object as an argument, e.g.:

["collator", {"case-sensitive": false, "diacritic-sensitive": true, "locale": "en"}]

This is a new pattern so we'll have to figure out the right iOS/macOS way to expose it.

/cc @1ec5 @friedbunny @julianrex

[1ec5]

There are a few pieces to implement here:

• Implement the c, d, and l modifiers to == and other operators: Many NSPredicate and NSExpression operators have no equivalent in style JSON #11786.
• When converting from JSON to NSPredicate, detect the use of a collator expression and add the c, d, and l modifiers as appropriate: Many NSPredicate and NSExpression operators have no equivalent in style JSON #11786.
• Update the documentation to note that these modifiers are supported.
• If collator can be used on its own, independently of a comparison expression, then we need to make sure it can be represented and round-tripped somehow – even if it’s just a struct in the NSPredicate syntax. But I think the modifiers would be the focus of our iOS/macOS support for collators, since it’s much more concise than what we could implement for standalone collators.

[ChrisLoer]

If collator can be used on its own, independently of a comparison expression, then we need to make sure it can be represented and round-tripped somehow – even if it’s just a struct in the NSPredicate syntax. But I think the modifiers would be the focus of our iOS/macOS support for collators, since it’s much more concise than what we could implement for standalone collators.

I'm not sure of all the terminology here, but currently a Collator (the result of evaluating a CollatorExpression) can be passed into comparison operators and it can also be passed as an argument to resolved-locale. It's not a valid result from an expression (e.g. there's no converter from Collator to mbgl::Value). cc @anandthakker

[1ec5]

OK, that makes sense. So the most common way to perform a case-insensitive, locale-sensitive comparison in Objective-C or Swift would be:

NSPredicate(format: "name_fr =[cl] 'quebec'")

Note how l means “current locale”; there isn’t a way to specify a specific locale using this syntax.

As it is, it’s already possible, if ugly, to create a collator expression manually like this:

NSPredicate(format: "MGL_FUNCTION('==', name_fr, 'quebec', MGL_FUNCTION('collator', %@))",
            ["case-sensitive": false, "diacritic-sensitive": true, "locale": "en"])

So far we’ve only defined aftermarket expression functions for the most common operators, but we could define one for creating a collator, with a syntax like this:

NSPredicate(format: "MGL_FUNCTION('==', name_fr, 'quebec',
                                  collatorCaseInsensitive:diacriticInsensitive:locale:(TRUE, FALSE, 'en'))")

But more likely, we’d have operators like == convert a dictionary in the third argument to a collator:

NSPredicate(format: "MGL_FUNCTION('==', name_fr, 'quebec', %@)",
            ["case-sensitive": false, "diacritic-sensitive": true, "locale": "en"])

If we encounter a collator expression in style JSON, we should be able to convert it to a native comparison in some cases, but if the collator specifies a locale, then we’d only be able to use the NSPredicate syntax if the locale happens to match the current locale. Otherwise, we’d have to use the more verbose syntax.

[1ec5]

The […] flags in NSPredicate are represented by NSComparisonPredicate.options.