The "is" keyword (or operator) has been available since Haxe 4.0, but it isn't mentioned at all in the manual. - [ ] Document "is" on the keyword page. It is not as restricted as the other keywords in that it is still usable as an identifier. - [ ] Document "is" on the binary operators page, in the last section next to the arrow operator.
The "is" keyword (or operator) has been available since Haxe 4.0, but it isn't mentioned at all in the manual.