It should be possible for an expression to test whether a string appears in an array or another string while ignoring case or diacritic distinctions, or while following the collation rules of a language other than the current language.

Problem

#6270 added an optional third argument to expression operators such as == and < to override the default collator when comparing two strings, for example to compare the strings case-insensitively or diacritic-insensitively. #8876 implements an in expression operator that streamlines comparing a string against a large or indefinite number of other strings. Unfortunately, this operator takes only two arguments and doesn’t accept a collator, so the developer has to choose between comparing case-insensitively or easily comparing multiple strings.

This issue is surfacing when porting in to NSExpression for iOS/macOS (mapbox/mapbox-gl-native-ios#168), because NSExpression supports [c] and [d] modifiers on the IN operator just as on the == and < operators.

Proposed design

The in expression operator should accept a collator object as an optional third argument:

["in", needle: (boolean, string or number), haystack: (array or string)]: boolean
["in", needle: (boolean, string or number), haystack: (array or string), collator]: boolean

The same issue affects the match operator, but it’s less clear where a collator object would go in a match expression without creating ambiguity for the expression evaluator.

Implementation notes

Here’s where the in comparisons currently take place:

Here’s how custom collators are evaluated in == expressions:

/ref #6484 mapbox/mapbox-gl-native#11786
/cc @mapbox/gl-js @chloekraw @fabian-guerra