Define: Structural Subtyping
Structural subtyping defines an implicit relationship between types that have the same structure.
Structural sub-typing in Haxe is allowed when unifying:
The following example is part of the Lambda class in the Haxe Standard Library:
public static function empty<T>(it : Iterable<T>):Bool { return !it.iterator().hasNext(); }
The empty-method checks if an Iterable has an element. For this purpose, it is not necessary to know anything about the argument type other than the fact that it is considered an iterable. This allows calling the empty-method with any type that unifies with Iterable<T>, which applies to many types in the Haxe Standard Library.
This kind of typing can be very convenient, but extensive use may be detrimental to performance on static targets, which is detailed in Impact on Performance.